托福阅读真题+题目+答案：The Extinction of the Dinosaurs

托福阅读真题+题目+答案：The Extinction of the Dinosaurs

1. Geologists define the boundary between sediment layers of the Cretaceous period (144..C65 million years ago) and the Paleocene period (65..C55 million years ago) in part by the types and amounts of rocks and fossils they contain or lack. Before the limit of 65 million years ago，marine

2. strata are rich in calcium carbonate due to accumulations of fossils of microscopic algae deposited on the sea floor. Above the 65-million-year limit，sea-floor sediments contain much less calcium carbonate ，and fossils of several families of mollusks are no longer found. In continental sediments ，dinosaur fossils ，though frequent before 65 million years ago ， are totally absent. By

3. contrast，new families of mammals appear，including large mammals for the first time. Scientists wondered for many years about what could have caused the dinosaurs' rapid disappearance at the end of the Cretaceous period，coming up with a great variety of theories and scenarios. For some, it could have been due to unfavorable genetic changes triggered by a dramatic increaseby a factor of 10，100，1，000 in cosmic-ray particles reaching the Earth after a supernova explosion somewhere in the neighborhood of the solar system. For these high-energy particles to affect life，they would have to get through the protective barrier of the Earth's magnetosphere ，the region of the upper atmosphere controlled by Earth's magnetic field. That could have happened if the cloud of particles from the supernova explosion reached the Earth during a period when the magnetosphere was weakened ， something that may happen when the Earth's magnetic field changes direction. And we know that the magnetic north and south poles of the Earth switch on the average twice every million years. However，this is not the only possible explanation for dinosaur destruction .

4. Other theories have raised the possibility of strong climate changes in the tropics (but they then must be explained) . Certainly，if climate changes，the changed distributions of temperature and rainfall modify the conditions that favor one ecosystem over another. The extinction of a particular family，genus，or species may result from a complicated chain of indirect causes and effects. Over thirty years ago ，scientist Carl Sagan quoted one suggestion that the demise of the dinosaurs resulted from the disappearance of a species of fern plant that was important for dinosaur digestion. Other theories involved a worldwide cold wave following the spread of a layer of cold but not very salty water in the world's oceans ，which floated on the surface because，with its low salinity，the water was less dense.

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托福阅读真题题目：

1..According to paragraph 1，which of the following is true of Paleocene sediments

A. They lack fossils from some families of mammals found in Cretaceous sediments.

B. They contain fossils of dinosaurs.

C. They contain fossils of some animals that did not exist during the Cretaceous.

D. They contain fossils of more kinds of mollusks than are found in Cretaceous sediments.

2..The word unfavorable in the passage is closest in meaning to

A. Unusual

B. Disadvantageous

C. Sudden

D. major

3..The word triggered in the passage is closest in meaning to

A. initiated

B. intensifie

C. followed

D. aided

4..Why does the author include the information that the magnetic north and south poles of the Earth switch on the average twice every million years

A. To provide evidence showing that Earth's protective barrier is difficult to get through

B. To show that it is reasonable to think that particles from a supernova explosion could have reached Earth

C. To explain why some scientists believe a supernova explosion may have occurred near our solar system

D. To help explain why some scientists have rejected the theory presented in paragraph 2

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