- 2017年12月08日14:05 来源：小站整理
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It is estimated that over 99 percent of all species that ever existed have become extinct. What causes extinction? When a species is no longer adapted to a changed environment, it may perish.
The exact causes of a species' death vary from situation to situation. Rapid ecological change may render an environment hostile to a species. For example, temperatures may change and a species may not be able to adapt. Food resources may be affected by environmental changes, which will then cause problems for a species requiring these resources. Other species may become better adapted to an environment, resulting in competition and, ultimately, in the death of a species.
The fossil record reveals that extinction has occurred throughout the history of Earth. Recent analyses have also revealed that on some occasions many species became extinct at the same time — a mass extinction. One of the best-known examples of mass extinction occurred 65 million years ago with the demise of dinosaurs and many other forms of life. Perhaps the largest mass extinction was the one that occurred 225 million years ago, when approximately 95 percent of all species died, mass extinctions can be caused by a relatively rapid change in the environment and can be worsened by the close interrelationship of many species. If, for example, something were to happen to destroy much of the plankton in the oceans, then the oxygen content of Earth would drop, affection even organisms not living in the oceans. Such a change would probably lead to a mass extinction.
One interesting, and controversial, finding is that extinctions during the past 250 million years have tended to be more intense every 26 million years. This periodic extinction might be due to intersection of the Earth's orbit with a cloud of comets, but this theory is purely speculative. Some researchers have also speculated that extinction may often be random. That is, certain species may be eliminated and others may survive for no particular reason. A species' survival may have nothing to do with its ability or inability to adapt. If so, some of evolutionary history may reflect a sequence of essentially random events.
1. The word "it" in line 3 refers to
(D) 99 percent
2. The word "ultimately" in line 8 is closest in meaning to
3. What does the author say in paragraph 1 regarding most species in Earth's history
(A) They have remained basically unchanged from their original forms.
(B) They have been able to adapt to ecological changes.
(C) They have caused rapid change in the environment.
(D) They are no longer in existence.
4. Which of the following is NOT mentioned in paragraph 1 as resulting from rapid ecological
(A) Temperature changes
(B)Availability of food resources
(C) Introduction of new species
(D) Competition among species
5. The word "demise" in line 12 is closest in meaning to
6. Why is "plankton" mentioned in line 17?
(A) To demonstrate the interdependence of different species.
(B) To emphasize the importance of food resources in preventing mass extinction.
(C) To illustrate a comparison between organisms that live on the land and those that live in the
(D) To point out that certain species could never become extinct.
7.According to paragraph 2, evidence from fossils suggests that
(A) Extinction of species has occurred from time to time throughout Earth's history.
(B) Extinctions on Earth have generally been massive
(C) There has been only one mass extinction in Earth's history.
(D) Dinosaurs became extinct much earlier than scientists originally believed.
8. The word "finding" in line 20 is closest in meaning to
(A) published information
(B) research method
(C) ongoing experiment
(D) scientific discovery
9. Which of the following can be inferred about the theory mentioned in Line 21-23?
(A) Many scientists could be expected to disagree with it.
(B) Evidence to support the theory has recently been found.
(C) The theory is no longer seriously considered.
(D) Most scientists believe the theory to be accurate.
10. In paragraph 3, the author makes which of the following statements about a species' survival?
(A) It reflects the interrelationship of many species.
(B) It may depend on chance events.
(C) It does not vary greatly from species to species
(D) It is associated with astronomical conditions.
11.According to the passage , it is believed that the largest extinction of a species occurred
(A) 26 million years ago
(B) 65 million years ago
(C) 225 million years ago
(D) 250 million years ago