The Sixth Extinction（灭绝） Extinctions，where entire species are wiped out，are not unusual in our Earth's history.In fact，over 90 percent of all species that ever lived are now extjye.aict.In the last 500 million years，there have been five times when life on Earth has almost ended.These include the third mass extinction，known as the"Great Dying"，which killed 90% to 96% of all species.And the fifth event，which occurred 66 million years ago，caused the death of the dinosaurs.The reasons for extinctions are varied.According to scientists，they include volcanic eruptions，asteroids （小行星） hitting the Earth from space，changes in sea levels，the decrease of the oxygen content of the sea and global warming.Many scientists say we are now entering the Earth's sixth mass extinction.This time，human activity will be to blame.
    Although it is not unusual for species to die out naturally，the rate at which this is now happening is cause for concern.A 2015 study by scientists who were based at Brown University and Duke University in the US，looked at how quickly species die out due to natural causes，that is，the "background extinction rate".They found that human activity is causing species to die out1，000 times faster than normal.There is a long list of reasons why so many species are dying out：air and water pollution，forests being destroyed，factory farming and overfishing.It is clear that human activity has negatively affected all other species on Earth，including animals and plants.
    If a sixth mass extinction occurs，scientists who have studied the issue believe that up to three quarters of all species on Earth could die out.Of course，as we humans depend on so many species for our survival，we would also be at risk of dying out.Our world and our lives depend on the balance in nature between animals and plants.Without forests，we would have no air to breathe and without clean water we would be unable to survive.
    Can a sixth extinction be avoided？Experts say that it is not yet too late，if we take action now.If we want to survive，we need to stop destroying the planet that we live on and start to protect it.

（1）What are the reasons for extinction in the history？

Volcanic eruptions，asteroids hitingthe Earth from space，changes in sea levels，the decrease of the oxygen content of the sea and global warming.

Volcanic eruptions，asteroids hitingthe Earth from space，changes in sea levels，the decrease of the oxygen content of the sea and global warming.

（2）Why does the writer believe we will probably die out if 3/4 of the Earth's species die out？

Because we humans depend on so many species for our survival.

Because we humans depend on so many species for our survival.

（3）Please decide which part is false in the following statement，then underline it and explain why.
It is not unusual for species to die out naturally，so we human beings are not to blame.

It is not unusual for species to die out naturally so we human beings are not to blame.Because humanactivity is causing species to die out I，000 times faster than normal，we human beings are to blame.

It is not unusual for species to die out naturally so we human beings are not to blame.Because humanactivity is causing species to die out I，000 times faster than normal，we human beings are to blame.

（4）What actions can we take to avoid the sixth extinction？ （In about 40 words）

We should try to avoid air and water pollution，forests destroying，factory farming and overfishing，because human activity has negatively affected all other species on Earth and is causing many species to die out.

We should try to avoid air and water pollution，forests destroying，factory farming and overfishing，because human activity has negatively affected all other species on Earth and is causing many species to die out.