【正确答案】[Topic] AS the professor mentions in the lecture, the mammoth and the mastodon are examples of animals that became extinct because of a climate change about 18,000 years ago. [Example 1] In the case of the mammoth, the professor shows how the warmer and wetter climate led to its extinction. The warmer and wetter conditions meant that trees could spread northwards, to where mammoths lived.These trees then took over and killed the smaller plants that mammoths ate, so then the mammoths also died because there was nothing left for them to eat. [Example 2] The other example she talks about is the mastodon, which also became extinct after the climate changed. The professor says that because the weather was warmer, the mastodon's habitat shrunk.The animals couldn't get enough to eat, so they started to die out as well.

【答案解析】[解析] W: During the Late Pleistocene stage of Earth's history, there was a period of mass extinctions that resulted in the loss of two thirds of all the large mammals on the planet. Animals like mammoths, saber-toothed cats, and mastodons disappeared during this extinction event. Now the cause of the Late Pleistocene extinctions isn't known for certain.., but some scientists are convinced that global climate change was primarily responsible. Between 18,000 and 15,000 years ago, climbing global temperatures may have initiated a period of mass extinctions. A researcher at the University of Alaska at Fairbanks says that the increase in moisture and the warmer temperatures caused many plant species to spread farther north, where mammoths primarily lived. You might think this was a good thing, since it would have given them a larger food supply. But what really happened was that the milder climate allowed trees to grow farther north too. And these trees out-competed the smaller plants that the mammoths ate. So their food supply quickly vanished, and with nothing to eat, the species vanished too. There's a paleontologist with the Denver Museum of Nature and Science who claims that the changing climate had a similar effect on mastodons. When the climate warmed up, the savannahs and the forests that mastodons lived in became less diverse. Not only that, they began to shrink. Of course, as that happened, mastodon populations themselves began to shrink too. You know, the mastodons couldn't find enough food or shelter to survive. Eventually, they totally died out, and all we're left with now are fossils.