2020年3月19日托福阅读真题+题目+答案：Extinction Trends

2020年3月19日托福阅读真题+题目+答案：Extinction Trends

In general, the diversity of life on Earth has been constant or increasing slightly over the past 200 million or 300 million years. ⬛ The geologic record shows new species emerging and old ones dying away, resulting in broad range of forms over geologic time. ⬛ On closer inspection,the geologic record also shows distinct short-term increases in the rate of extinction termed extinction events. ⬛ One such event occurred 65 million years ago and marked the end of the Mesozoic Era, the age of reptiles. ⬛ For years, the cause of this event puzzled scientists. However, in the980 s thin layer of unique sediment containing the element iridium was discovered in sedimentary bedrock whose age coincided with this event. Research showed that this sediment was created by an explosion on Earth's surface, probably caused by an asteroid impact. The explosion threw a huge mass of debris into the atmosphere, which, scientists reason reduced incoming solar radiation and cooled Earth. Plants declined and the food sources for many herbivores (plant-eating animals) apparently disappeared. followed shortly thereafter by a sequence of extinctions among predators.

1.In paragraph 1, why does the author mention the discovery of a layer of unique sediment containing iridium?

O To explain why scientists were puzzled by the extinction event at the end of the Mesozoic Era

O To suggest that the presence of iridium probably caused the decline of plants and food resources

O To identify evidence that led to better understanding of major extinction event in Earth's history

O To provide an example of the type of sediment that is associated with explosions on Earth's surface

2.Paragraph 1 suggests that many predators became extinct at the end of the Mesozoic Era because

O their bodies could not survive the extreme cold due to the lack of solar radiation

O they were killed by the impact of an explosion on Earth's surface

O a reduction in plant food sources had reduced the amount of available prey

O an asteroid that hit Earth released poisonous elements into the atmosphere

Many other extinction events are also identifiable in the geologic record. Some occur at 26-million-year intervals, and scientists speculate that they may be associated with extraterrestrial impacts similar to the one at the end of the Mesozoic Era. Between these events, many smaller episodes of extinction are also apparent. The influence of humans on extinction patterns. however. does not appear until about 10.000 to 15.000 years ago.

3.Which of the following is true of the extinction events discussed in paragraph 2?

O They were all result of extraterrestrial impacts.

O They seem to have stopped occurring about 10,000 years ago.

O They became steadily more frequent over time.

O They were unconnected with human activity until about 15,000 years ago.

The last glaciation on Earth reached its maximum 15,000 to 20.000 years ago. At this time the northern half of North America was covered with great sheets of glacial ice. A similarly large mass of ice covered large part of northern Eurasia. Humans lived in the zone south of the ice border, where they survived by hunting and gathering. Among the animals hunted were large mammals such as mammoths and mastodons. which are now extinct. The extinction process appears to have taken place over several thousand years in the presence of humans as the ice sheets were melting and the bioclimatic environment was undergoing change. By 8,000 to 10,000 years ago, dozens of mammal species had disappeared. Scientists debate whether the extinctions were driven mainly by changing climate or mainly by human exploitation. No doubt these animals were under stress from a changing environment. but the evidence for population reduction by early hunters is also compelling. On balance, it appears certain that humans had a hand in the process. At the very least, hunting hastened the extinction process.

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