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　　There have been numerous well-documented extinctions of indigenous species caused by the introduction of nonindigenous predators and pathogens. However, surprisingly few extinctions of indigenous species can be attributed to competition from introduced species. For example, during the past 400 years, 4,000 plant species have been introduced into North America, and these nonindigenous plants currently account for nearly 20 percent of North America’s plant species. Yet no evidence exists that any indigenous North American plant species became extinct as a result of competition from introduced plant species. The scarcity of documented extinctions caused by competition from new species could mean that such extinctions take longer to occur than scientists initially believed or, alternatively, that extinctions are rarely caused by competition from nonindigenous species.

　　1. The passage is concerned primarily with

　　A：Pointing out that a particular type of species extinction is rarely known to occur

　　B：Proposing a possible explanation for conflicting data about a particular type of species extinction

　　C：Resolving a debate about the frequency of a particular type of species extinction

　　D：Comparing two theories regarding possible causes of a particular type of species extinction

　　E：Refuting a proposed explanation for the increasingly rare occurrence of a particular type of species extinction

　　答案：A

　　2. The author introduces statistics about North America’s nonindigenous plant species primarily in order to

　　A：undermine a proposed explanation for the absence of any evidence for the occurrence of a particular phenomenon.

　　B：contrast the effect of introduced plant species in North America with the effect that introduced animal species have had.

　　C：suggest that North America’s indigenous plants are a domain in which there has been ample scope for a particular effect to have occurred.

　　D：emphasize how much the ecology of North America has been affected over the past 400 years by the introduction of nonindigenous species

　　E：substantiate a claim about the overall effect that the introduction of nonindigenous species tends to have on indigenous populations.

　　答案：E

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