2. 外语
  3. 　　　 It is estimated that over 99 percent of all species that ever existed have become 　　　 extinct. What causes extinction?

　　　 It is estimated that over 99 percent of all species that ever existed have become 　　　 extinct. What causes extinction?

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问题 　　　     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.
(5)　   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.
(10)　  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
(15)　  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.
(20)　　    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 tat extinction
　　　 may often be random. That is, certain species may be eliminated and others may survive
(25)　  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.
选项 A、environment
B、species
C、extinction
D、99 percent
答案B
解析
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