新世界交互英语(第二版)视听说4 Unit 2 Survival 参考答案 -2/44

One scientific study suggests that up to two billion different species may be alive now. That's a big number. However, since life began on Earth, scientists think at least five billion kinds of plants and animals have gone extinct. They know this from the discovery of : dead plants or animals that have turned to stone.
Why did so many species not survive? Some species because conditions changed. Others were killed in a huge natural disaster that happened about 66 million years ago. A big rock from space, called , hit Earth. It killed the dinosaurs and caused . If humans had been alive at that time, they would have felt terror.
Many species are these days, so some scientists think another mass extinction is happening now. They see a relationship between human activity and these deaths. For example, they think the problem is happening because humans are . Is there any way to rescue species that are in danger? Maybe. If a scientist found a way to stop climate change, he or she would be a hero to people who care about the planet.

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