問題詳情

Question 20-30It is estimated that over 99 percent of all species that ever existed have becomeextinct. What causes extinction? When a species is no longer adapted to a changedenvironment, it may perish. The exact causes of a species’ death vary from situationLine 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. FoodResources may be affected by environmental changes, which will then cause problemsFor a species requiring these resources. Other species may become better adapted to anEnvironment, 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 extinctat the same time—a mass extinction. One of the best-known examples of mass extinctionoccurred 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 closeinterrelationship of many species. If, for example, something were to happen to destroymuch 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 250Million years have tended to be more intense every 26 million years. This periodicextinction might be due to intersection of the Earth’s orbit with a cloud of comets, butthis theory is purely speculative. Some researchers have also speculated tat extinctionmay 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 orinability to adapt. If so, some of evolutionary history may reflect a sequence of essentiallyrandom events.
20. The word “it” in line 3 refers to
(A) environment
(B) species
(C) extinction
(D) 99 percent

參考答案

答案：B
難度：非常簡單1
統計：A(0),B(12),C(0),D(0),E(0)

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