单选题Although many people would not believe it, the mosquito is actually the most dangerous animal in Africa. While the bite of the black mamba is invariably lethal when untreated, this dreaded snake kills only a few dozen people per year. Hippopotami, with

共用题干
Eastern Quakes can Trigger Big Shakes

In the first week of November 2011,people in central Oklahoma experienced more than two dozen earth-
quakes.The largest,a magnitude（量）5.6 quake,shook thousands of fans in a college football stadium,
caused cracks in a few buildings and scared many people who had never felt a quake before.Oklahoma is not
an area of the country famous for its quakes.If you watch the news on TV,you will see reports about all sorts
of natural disasters.But the most dangerous type of natural disaster,and also the most unpredictable,is the
earthquake.
Researchers at the US Geological Survey(USGS)estimate that several million earthquakes occur globally
each year. That may sound scary,but people don't feel many of them because they happen in remote and
unpopulated regions.Many quakes happen under the ocean,and others have a very small magnitude.
Scientists know about small,remote quakes only because of very sensitive electronic devices called seis-
mometers（地震仪）.These devices detect and measure the size of ground vibrations（震颤）produced by
earthquakes.Altogether,USGS researchers use seismometers to identify and locate about 20,000 earthquakes
each year.
Although earthquakes can happen anywhere in the world,really big quakes occur only in certain areas.
The largest ones register a magnitude 8 or higher and happen,on average,only once each year. Such big
ones typically occur along the edges of Earth ' s tectonic plates（构造板块）.
Tectonic plates are huge pieces of Earth ' s crust（外壳）,sometimes many kilometers thick. Often,
edges of these plates temporarily lock together. When plates push and scrape（擦）past each other earthquakes
occur. On average,tectonic plates move very slowly一about the same speed as your fingernails grow.
But sometimes earthquakes rumble（轰轰作响）through portions of the landscape far from a plate ' s
edges.Although less expected,these"mid-plate"small earthquakes can do substantial damage.Some of the
biggest known examples hit the eastern half of the United States two centuries ago.Today,scientists are still
puzzling over why the quakes occurred and when similar ones might occur.

The earthquake is the most unpredictable natural disaster.
A:Right
B:Wrong
C:Not mentioned

共用题干
Eastern Quakes can Trigger Big Shakes

In the first week of November 2011,people in central Oklahoma experienced more than two dozen earth-
quakes.The largest,a magnitude（量）5.6 quake,shook thousands of fans in a college football stadium,
caused cracks in a few buildings and scared many people who had never felt a quake before.Oklahoma is not
an area of the country famous for its quakes.If you watch the news on TV,you will see reports about all sorts
of natural disasters.But the most dangerous type of natural disaster,and also the most unpredictable,is the
earthquake.
Researchers at the US Geological Survey(USGS)estimate that several million earthquakes occur globally
each year. That may sound scary,but people don't feel many of them because they happen in remote and
unpopulated regions.Many quakes happen under the ocean,and others have a very small magnitude.
Scientists know about small,remote quakes only because of very sensitive electronic devices called seis-
mometers（地震仪）.These devices detect and measure the size of ground vibrations（震颤）produced by
earthquakes.Altogether,USGS researchers use seismometers to identify and locate about 20,000 earthquakes
each year.
Although earthquakes can happen anywhere in the world,really big quakes occur only in certain areas.
The largest ones register a magnitude 8 or higher and happen,on average,only once each year. Such big
ones typically occur along the edges of Earth ' s tectonic plates（构造板块）.
Tectonic plates are huge pieces of Earth ' s crust（外壳）,sometimes many kilometers thick. Often,
edges of these plates temporarily lock together. When plates push and scrape（擦）past each other earthquakes
occur. On average,tectonic plates move very slowly一about the same speed as your fingernails grow.
But sometimes earthquakes rumble（轰轰作响）through portions of the landscape far from a plate ' s
edges.Although less expected,these"mid-plate"small earthquakes can do substantial damage.Some of the
biggest known examples hit the eastern half of the United States two centuries ago.Today,scientists are still
puzzling over why the quakes occurred and when similar ones might occur.

Seismometers can identify and locate most of the earthquakes in China.
A:Right
B:Wrong
C:Not mentioned

共用题干
Prolonging Human Life

1.Prolonging human life has increased the size of the human population.Many people alive today would have died of childhood diseases if they had been born 100 years ago.Because more people live longer,there are more people around at any given time.In fact,it is a decrease in death rates,not an increase in birthrates,that has led to the population explosion.
2.Prolonging human life has also increased the dependency load.In all societies,people who are disabled or too young or too old to work are dependent on the rest of society to provide for them. In hunting and gathering cultures,old people who could not keep up might be left behind to die.In times of famine,infants might be allowed to die because they could not survive if their parents starved,whereas if the parents survived they could have another child.
3.In most contemporary societies,people feel a moral obligation to keep people alive whether they can work or not. We have a great many people today who live past the age at which they want to work or are able to work.We also have rules which require people to retire at a certain age.Unless these people were able to save money for their retirement,somebody else must support them.In the United States many retired people live on social security checks which are so little that they must live in near poverty. Older people have more illness than young or middle-aged people.Unless they have wealth or private or government insurance,they must often"go on welfare"if they have a serious illness.
4.When older people become senile or too weak and ill to care for themselves,they create grave problems for their families.In the past and in some traditional cultures,they would be cared for at home until they died.Today,with most members of a household working or in school,there is often no one at home who can care for a sick or weak person.To meet this need,a great many nursing homes and convalescent hospitals have been built. These are often profit-making organizations, although some are sponsored by religious and other nonprofit groups.While a few of these institutions are good,most of them are simply"dumping grounds"for the dying in which"care"is given by poorly paid,overworked,and under-skilled personnel.

Many people would have died of_______if they had been born 100 years ago.
A:too weak and ill
B:hunting and gathering cultures
C:childhood diseases
D:no one at home
E:epidemic diseases
F: traditional cultures

共用题干
Prolonging Human Life
Prolonging human life has increased the size of the human population. Many people alive
today would have died of childhood diseases if they had been born 100 years ago.Because
more people live longer,there are more people around at any given time. In fact,it is a decrease in death rates,not an increase in birthrates,that has led to the population explosion.
Prolonging human life has also increased the dependency load. In all societies,people who are disabled or too young or too old to work are dependent on the rest of society to provide for them. In hunting and gathering cultures,old people who could not keep up might be left behind to die. In times of famine,infants might be allowed to die because they could not survive if their parents starved,whereas if the parents survived they could have another child. In most contemporary societies,people feel a moral obligation to keep people alive whether they can work or not. We have a great many people today who live past the age at which they want to work or are able to work;we also have rules which require people to retire at a certain age. Unless these people were able to save money for their retirement,some-body else must support them. In the United States many retired people live on social security checks which are so little that they must live in near poverty. Older people have more illness than young or middle-aged people;unless they have wealth or private or government insur-ance,they must often“go on welfare” if they have a serious illness.
When older people become senile or too weak and ill to care for themselves,they create grave problems for their families. In the past and in some traditional cultures,they would be cared for at home until they died. Today,with most members of a household working or in school,there is often no one at home who can care for a sick or weak person. To meet this need,a great many nursing homes and convalescent hospitals have been built.These are of-ten profit-making organizations,although some are sponsored by religious and other nonprofit groups.While a few of these institutions are good,most of them are simply“dumping grounds”for the dying in which“care”is given by poorly paid,overworked,and under-skilled personnel.

According to the passage,which of the following statements about retired people in the United States is true?
A: Many of them have a very hard life.
B: They cannot live a decent life without enough bank savings.
C: They rely mainly on their children for financial support.
D: Most of them live with their children and therefore are well looked after.

共用题干
Eastern Quakes Can Trigger Big Shakes
In the first week of November 2011,people in central Oklahoma experienced more than two dozen earthquakes. The largest,a magnitude 5 .6 quake,shook thousands of fans in a college football stadium,caused cracks in a few buildings and rattled the nerves of many people who had never felt a quake before. Oklahoma is not an area of the country famous for its quakes. If you watch the news on TV,you see reports about all sorts of natural disasters一 hurricanes,tornadoes,flooding and wildfires,to name a few. But the most dangerous type of natural disaster,and also the most unpredictable,is the earthquake.
Researchers at the U. S. Geological Survey estimate that several million earthquakes rattie the globe each year. That may sound scary,but people don't feel many of the tremors because they happen in remote and unpopulated regions.Many quakes happen under the ocean, and others have a very small magnitude,or shaking intensity.
A magnitude 5.8 earthquake that struck central Virginia the afternoon of August 23, 2011,was felt from central Georgia to southeastern Canada. In many urban areas,including Washington,D.C.,and New York City(Wall Street shown),people crowded the streets while engineers inspected buildings.
Scientists know about small,remote quakes only because of very sensitive electronic devices called seismometers.These devices detect and measure the size of ground vibrations
produced by earthquakes.Altogether,USGS researchers use seismometers to identify and locate about 20,000 earthquakes each year.
Although earthquakes can happen anywhere in the world,really big quakes occur only in certain areas. The largest ones register a magnitude 8 or higher and happen,on average,only once each year. Such big ones typically occur along the edges of Earth's tectonic plates.
Tectonic plates are huge pieces of Earth's crust,sometimes many kilometers thick. These plates cover our planet's surface like a jigsaw puzzle. Often,jagged edges of these plates temporarily lock together. When plates jostle and scrape past each other,earthquakes occur. On average,tectonic plates move very slowly一about the same speed as your fingernails grow.
But sometimes earthquakes rumble through portions of the landscape far from a plate's edges. Although less expected,these”mid-plate” tremors can do substantial damage. Some of the biggest known examples rattled the eastern half of the United States two centuries ago. Today,scientists are still puzzling over why the quakes occurred and when similar ones might occur.

Seismometers can identify and locate most of the earthquakes in China.
A: Right
B: Wrong
C: Not mentioned

共用题干
Prolonging Human Life
Prolonging human life has increased the size of the human population.Many people alive today would have died of childhood diseases if they had been born 100 years ago.Because more people live longer,there are more people around at any given time.In fact,it is a decrease in death rates,not an increase in birthrates,that has led to the population explosion.
Prolonging human life has also increased the dependency load.In all societies,people who are disabled or too young or too old to work are dependent on the rest of society to provide for them.In hunting and gathering cultures, old people who could not keep up might be left behind to die.In times of famine,infants might be allowed to die because they could not survive if their parents starved,whereas if the parents survived they could have another child.In most contemporary societies,people feel a moral obligation to keep people alive whether they can work or not.We have a great many people today who live past the age at which they want to work or are able to work;we also have rules which require people to retire at a certain age.Unless these people were able to save money for their retirement,somebody else must support them.In the United States many retired people live on social security checks which are so little that they must live in near poverty.Older people have more illness than young or middle-aged people;unless they have wealth or private or government insurance,they must often"go on welfare"if they have a serious illness.
When older people become senile or too weak and ill to care for themselves,they create grave problems for their families.In the past and in some traditional cultures,they would be cared for at home until they died.Today,with most members of a household working or in school,there is often no one at home who can care for a sick or weak person.To meet this need,a great many nursing homes and convalescent hospitals have been built.These are often profit-making organizations,although some are sponsored by religious and other nonprofit groups.While a few of these institutions are good,most of them are simply"dumping grounds"for the dying in which"care"is given by poorly paid,overworked,and underskilled personnel.

The writer believes that the population explosion results from______.
A:an increase in birthrates
B:the industrial development
C:a decrease in death rates
D:cultural advances

共用题干
Eastern Quakes can Trigger Big Shakes

In the first week of November 2011,people in central Oklahoma experienced more than two dozen
earthquakes.The largest,a magnitude 5.6 quake,shook thousands of fans in a college football itadium,
caused cracks in a few buildings and rattled the nerves of many people who had never felt a quake before.
Oklahoma is not an area of the country famous for its quakes.If you watch the news on TV,you will see
reports about all sorts of natural disasters一hurricanes,tornadoes,flooding and wildfires,to name a few.But
the most dangerous type of natural disaster,and also the most unpredictable,is the earthquake.
Researchers at the U.S.Geological Survey estimate that several million earthquakes rattle the globe each
year. That mnay sound scary,but people don'I feel many of the tremors because they happen in remote and
unpopulated regions.Many quakes happen under the ocean,and othlers have a very small magnitude,or
shaking intensity..
A magnitude 5.8 earthquake that struck central Virginia the afternoon of August 23,2011,was felt
from central Georgia to southeastern Canada.In iilany urbanl areas,including Washington,D.C.,dnd New
York City(Wall Street shown),people crowded the streets while engineer inspected buildings.Credit:
Wikimnedia/Alex Tahak.
Scientists know about small,remote quakes only because of very sensitive electronic devices called seis-
mometers.These devices detect and measure the size of ground vibrations produced by earthquakes.
Altogether,USGS researchers use seismornetero to identify and locate about 20,000 earthquaks
each year.
Although earthquakes can happen anywhere in the world,really big quakeg occur only in。erttiin areas.
The largest ones register a magnitude S or higher and happen, on average,only once each year. Such big
ones typically occur along the edges of Earth’s tectonic plates.
Tectonic plates are huge pieces of Earth’s crust,sornetinies many kilometers thick.These plates cover
our planet's
surface ike a jigsaw puzzle.Often,jagged edges of these plates temporarily lock together. When
riates jostle and serape past each other earthquakes occur. On。、crage,tectonic plates move very slowly一
about the same speed as sour fingernails grow.
But sometimes earthquakes rumble through portions of the landscape far from a plate'S edges.Although
less cxpcctcd,these"mid-plate"tremors can do substontaI damage,Some of the biggest known examples
rattled the eastern haif of the United States two centuries ago.Today,scientists are still puzzling over why the
quakes occurred and when similar ones nught occur.

The earthquake that hit the eastern half of the United States two centuries ago is the biggest"mid-plate"one in history.
A:Right
B:Wrong
C:Not mentioned

While some office jobs would seem boring to many people,there are quite a few jobs that are stimulating,exciting and satisfying.

A:hostile
B:tedious
C:fantastic
D:courageous

共用题干
Prolonging Human Life
Prolonging human life has increased the size of the human population. Many people alive
today would have died of childhood diseases if they had been born 100 years ago.Because
more people live longer,there are more people around at any given time. In fact,it is a decrease in death rates,not an increase in birthrates,that has led to the population explosion.
Prolonging human life has also increased the dependency load. In all societies,people who are disabled or too young or too old to work are dependent on the rest of society to provide for them. In hunting and gathering cultures,old people who could not keep up might be left behind to die. In times of famine,infants might be allowed to die because they could not survive if their parents starved,whereas if the parents survived they could have another child. In most contemporary societies,people feel a moral obligation to keep people alive whether they can work or not. We have a great many people today who live past the age at which they want to work or are able to work;we also have rules which require people to retire at a certain age. Unless these people were able to save money for their retirement,some-body else must support them. In the United States many retired people live on social security checks which are so little that they must live in near poverty. Older people have more illness than young or middle-aged people;unless they have wealth or private or government insur-ance,they must often“go on welfare” if they have a serious illness.
When older people become senile or too weak and ill to care for themselves,they create grave problems for their families. In the past and in some traditional cultures,they would be cared for at home until they died. Today,with most members of a household working or in school,there is often no one at home who can care for a sick or weak person. To meet this need,a great many nursing homes and convalescent hospitals have been built.These are of-ten profit-making organizations,although some are sponsored by religious and other nonprofit groups.While a few of these institutions are good,most of them are simply“dumping grounds”for the dying in which“care”is given by poorly paid,overworked,and under-skilled personnel.

Which of the following best describes the writer's attitude toward most of the nursing homes,and convalescent hospitals?
A: Sympathetic.
B: Unfriendly.
C: Optimistic.
D: Critical.

共用题干
Prolonging Human Life

1.Prolonging human life has increased the size of the human population.Many people alive today would have died of childhood diseases if they had been born 100 years ago.Because more people live longer,there are more people around at any given time.In fact,it is a decrease in death rates,not an increase in birthrates,that has led to the population explosion.
2.Prolonging human life has also increased the dependency load.In all societies,people who are disabled or too young or too old to work are dependent on the rest of society to provide for them. In hunting and gathering cultures,old people who could not keep up might be left behind to die.In times of famine,infants might be allowed to die because they could not survive if their parents starved,whereas if the parents survived they could have another child.
3.In most contemporary societies,people feel a moral obligation to keep people alive whether they can work or not. We have a great many people today who live past the age at which they want to work or are able to work.We also have rules which require people to retire at a certain age.Unless these people were able to save money for their retirement,somebody else must support them.In the United States many retired people live on social security checks which are so little that they must live in near poverty. Older people have more illness than young or middle-aged people.Unless they have wealth or private or government insurance,they must often"go on welfare"if they have a serious illness.
4.When older people become senile or too weak and ill to care for themselves,they create grave problems for their families.In the past and in some traditional cultures,they would be cared for at home until they died.Today,with most members of a household working or in school,there is often no one at home who can care for a sick or weak person.To meet this need,a great many nursing homes and convalescent hospitals have been built. These are often profit-making organizations, although some are sponsored by religious and other nonprofit groups.While a few of these institutions are good,most of them are simply"dumping grounds"for the dying in which"care"is given by poorly paid,overworked,and under-skilled personnel.

When older people become_______to care for themselves,they create grave problems for their families.
A:too weak and ill
B:hunting and gathering cultures
C:childhood diseases
D:no one at home
E:epidemic diseases
F: traditional cultures

共用题干
Eastern Quakes Can Trigger Big Shakes
In the first week of November 2011,people in central Oklahoma experienced more than two dozen earthquakes. The largest,a magnitude 5 .6 quake,shook thousands of fans in a college football stadium,caused cracks in a few buildings and rattled the nerves of many people who had never felt a quake before. Oklahoma is not an area of the country famous for its quakes. If you watch the news on TV,you see reports about all sorts of natural disasters一 hurricanes,tornadoes,flooding and wildfires,to name a few. But the most dangerous type of natural disaster,and also the most unpredictable,is the earthquake.
Researchers at the U. S. Geological Survey estimate that several million earthquakes rattie the globe each year. That may sound scary,but people don't feel many of the tremors because they happen in remote and unpopulated regions.Many quakes happen under the ocean, and others have a very small magnitude,or shaking intensity.
A magnitude 5.8 earthquake that struck central Virginia the afternoon of August 23, 2011,was felt from central Georgia to southeastern Canada. In many urban areas,including Washington,D.C.,and New York City(Wall Street shown),people crowded the streets while engineers inspected buildings.
Scientists know about small,remote quakes only because of very sensitive electronic devices called seismometers.These devices detect and measure the size of ground vibrations
produced by earthquakes.Altogether,USGS researchers use seismometers to identify and locate about 20,000 earthquakes each year.
Although earthquakes can happen anywhere in the world,really big quakes occur only in certain areas. The largest ones register a magnitude 8 or higher and happen,on average,only once each year. Such big ones typically occur along the edges of Earth's tectonic plates.
Tectonic plates are huge pieces of Earth's crust,sometimes many kilometers thick. These plates cover our planet's surface like a jigsaw puzzle. Often,jagged edges of these plates temporarily lock together. When plates jostle and scrape past each other,earthquakes occur. On average,tectonic plates move very slowly一about the same speed as your fingernails grow.
But sometimes earthquakes rumble through portions of the landscape far from a plate's edges. Although less expected,these”mid-plate” tremors can do substantial damage. Some of the biggest known examples rattled the eastern half of the United States two centuries ago. Today,scientists are still puzzling over why the quakes occurred and when similar ones might occur.

The earthquake that hit the eastern half of the United States two centuries ago is the biggest“mid-plate” one in history.
A: Right
B: Wrong
C: Not mentioned

共用题干
Eastern Quakes Can Trigger Big Shakes
In the first week of November 2011,people in central Oklahoma experienced more than two dozen earthquakes. The largest,a magnitude 5 .6 quake,shook thousands of fans in a college football stadium,caused cracks in a few buildings and rattled the nerves of many people who had never felt a quake before. Oklahoma is not an area of the country famous for its quakes. If you watch the news on TV,you see reports about all sorts of natural disasters一 hurricanes,tornadoes,flooding and wildfires,to name a few. But the most dangerous type of natural disaster,and also the most unpredictable,is the earthquake.
Researchers at the U. S. Geological Survey estimate that several million earthquakes rattie the globe each year. That may sound scary,but people don't feel many of the tremors because they happen in remote and unpopulated regions.Many quakes happen under the ocean, and others have a very small magnitude,or shaking intensity.
A magnitude 5.8 earthquake that struck central Virginia the afternoon of August 23, 2011,was felt from central Georgia to southeastern Canada. In many urban areas,including Washington,D.C.,and New York City(Wall Street shown),people crowded the streets while engineers inspected buildings.
Scientists know about small,remote quakes only because of very sensitive electronic devices called seismometers.These devices detect and measure the size of ground vibrations
produced by earthquakes.Altogether,USGS researchers use seismometers to identify and locate about 20,000 earthquakes each year.
Although earthquakes can happen anywhere in the world,really big quakes occur only in certain areas. The largest ones register a magnitude 8 or higher and happen,on average,only once each year. Such big ones typically occur along the edges of Earth's tectonic plates.
Tectonic plates are huge pieces of Earth's crust,sometimes many kilometers thick. These plates cover our planet's surface like a jigsaw puzzle. Often,jagged edges of these plates temporarily lock together. When plates jostle and scrape past each other,earthquakes occur. On average,tectonic plates move very slowly一about the same speed as your fingernails grow.
But sometimes earthquakes rumble through portions of the landscape far from a plate's edges. Although less expected,these”mid-plate” tremors can do substantial damage. Some of the biggest known examples rattled the eastern half of the United States two centuries ago. Today,scientists are still puzzling over why the quakes occurred and when similar ones might occur.

Few earthquakes happen without people's awareness.
A: Right
B: Wrong
C: Not mentioned

共用题干
Eastern Quakes can Trigger Big Shakes

In the first week of November 2011,people in central Oklahoma experienced more than two dozen
earthquakes.The largest,a magnitude 5.6 quake,shook thousands of fans in a college football itadium,
caused cracks in a few buildings and rattled the nerves of many people who had never felt a quake before.
Oklahoma is not an area of the country famous for its quakes.If you watch the news on TV,you will see
reports about all sorts of natural disasters一hurricanes,tornadoes,flooding and wildfires,to name a few.But
the most dangerous type of natural disaster,and also the most unpredictable,is the earthquake.
Researchers at the U.S.Geological Survey estimate that several million earthquakes rattle the globe each
year. That mnay sound scary,but people don'I feel many of the tremors because they happen in remote and
unpopulated regions.Many quakes happen under the ocean,and othlers have a very small magnitude,or
shaking intensity..
A magnitude 5.8 earthquake that struck central Virginia the afternoon of August 23,2011,was felt
from central Georgia to southeastern Canada.In iilany urbanl areas,including Washington,D.C.,dnd New
York City(Wall Street shown),people crowded the streets while engineer inspected buildings.Credit:
Wikimnedia/Alex Tahak.
Scientists know about small,remote quakes only because of very sensitive electronic devices called seis-
mometers.These devices detect and measure the size of ground vibrations produced by earthquakes.
Altogether,USGS researchers use seismornetero to identify and locate about 20,000 earthquaks
each year.
Although earthquakes can happen anywhere in the world,really big quakeg occur only in。erttiin areas.
The largest ones register a magnitude S or higher and happen, on average,only once each year. Such big
ones typically occur along the edges of Earth’s tectonic plates.
Tectonic plates are huge pieces of Earth’s crust,sornetinies many kilometers thick.These plates cover
our planet's
surface ike a jigsaw puzzle.Often,jagged edges of these plates temporarily lock together. When
riates jostle and serape past each other earthquakes occur. On。、crage,tectonic plates move very slowly一
about the same speed as sour fingernails grow.
But sometimes earthquakes rumble through portions of the landscape far from a plate'S edges.Although
less cxpcctcd,these"mid-plate"tremors can do substontaI damage,Some of the biggest known examples
rattled the eastern haif of the United States two centuries ago.Today,scientists are still puzzling over why the
quakes occurred and when similar ones nught occur.

Few earthquakes happen without people’s awareness.
A:Right
B:Wrong
C:Not mentioned

共用题干
Eastern Quakes can Trigger Big Shakes

In the first week of November 2011,people in central Oklahoma experienced more than two dozen
earthquakes.The largest,a magnitude 5.6 quake,shook thousands of fans in a college football itadium,
caused cracks in a few buildings and rattled the nerves of many people who had never felt a quake before.
Oklahoma is not an area of the country famous for its quakes.If you watch the news on TV,you will see
reports about all sorts of natural disasters一hurricanes,tornadoes,flooding and wildfires,to name a few.But
the most dangerous type of natural disaster,and also the most unpredictable,is the earthquake.
Researchers at the U.S.Geological Survey estimate that several million earthquakes rattle the globe each
year. That mnay sound scary,but people don'I feel many of the tremors because they happen in remote and
unpopulated regions.Many quakes happen under the ocean,and othlers have a very small magnitude,or
shaking intensity..
A magnitude 5.8 earthquake that struck central Virginia the afternoon of August 23,2011,was felt
from central Georgia to southeastern Canada.In iilany urbanl areas,including Washington,D.C.,dnd New
York City(Wall Street shown),people crowded the streets while engineer inspected buildings.Credit:
Wikimnedia/Alex Tahak.
Scientists know about small,remote quakes only because of very sensitive electronic devices called seis-
mometers.These devices detect and measure the size of ground vibrations produced by earthquakes.
Altogether,USGS researchers use seismornetero to identify and locate about 20,000 earthquaks
each year.
Although earthquakes can happen anywhere in the world,really big quakeg occur only in。erttiin areas.
The largest ones register a magnitude S or higher and happen, on average,only once each year. Such big
ones typically occur along the edges of Earth’s tectonic plates.
Tectonic plates are huge pieces of Earth’s crust,sornetinies many kilometers thick.These plates cover
our planet's
surface ike a jigsaw puzzle.Often,jagged edges of these plates temporarily lock together. When
riates jostle and serape past each other earthquakes occur. On。、crage,tectonic plates move very slowly一
about the same speed as sour fingernails grow.
But sometimes earthquakes rumble through portions of the landscape far from a plate'S edges.Although
less cxpcctcd,these"mid-plate"tremors can do substontaI damage,Some of the biggest known examples
rattled the eastern haif of the United States two centuries ago.Today,scientists are still puzzling over why the
quakes occurred and when similar ones nught occur.

The earthquake is the most unpredictable natural disaster.
A:Right
B:Wrong
C:Not rnentioned