托福阅读真题+题目+答案：Cretaceous Extinctions

托福阅读真题+题目+答案：Cretaceous Extinctions

Even though the timing of plant and animal extinctions is usually impossible to determine precisely, it seems that an extraordinary number of organisms on land and in the oceans went extinct at the very end of the Cretaceous period (145 million to 66 million years ago). ▊ Popular attention to this event long focused on the extinction of the dinosaurs, ignoring the simultaneous marine catastrophe. ▊ Various hypotheses were offered as explanation, such as widespread disease, overpredation, drastic climate change, loss of habitat, or lethal doses of radiation from a supernova occurring nearby. ▊ But most such explanations suffered from failing to explain the apparent suddenness of extinction of so many different groups of organisms in widely varying habitats or lacked support from the geological record. ▊

1.The word “lethal” in the passage is closest in meaning to

A.deadly

B.high

C.repeated

D.sudden

2.According to paragraph 1, what was one of the reasons why many of the hypotheses about the mass extinctions were considered unsatisfactory?

A.They could not determine the precise timing of the extinctions.

B.They exaggerated the suddenness of the extinctions.

C.They did not explain why only certain types of habitat were affected.

D.They were not supported by geological evidence.

In 1980, a team of scientists led by Nobel laureate Luis Alvarez and his son Walter published an article on a thin layer of clay found in marine sediments at the boundary between the layers formed during the Cretaceous and Tertiary (66 million to 2,588 million years ago) periods, commonly referred to as the K-T boundary, in several locations around the world. When analyzed, this clay layer was found to contain levels of the element indium, a heavy metallic element that is rare in Earth's crust but more concentrated in metallic meteorites, at more than ten times the normal concentration. The scientists reasoned that the clay was deposited from a world-encircling layer of dust enriched in iridium created a result of the impact of a big piece of space debris. Their calculations suggested that a collision with an asteroid (a small rocky body orbiting the Sun) with a diameter of at least ten kilometers was responsible for this dust layer.

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