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    本题由lindy提供
3007次学习

More selective than most chemical pesticides in that they ordinarily destroy only unwanted species, biocontrol agents (such as insects, fungi,

and viruses) eat, infect, or parasitize targeted plant or animal pests. However, biocontrol agents can negatively affect nontarget species by, for example, competing with them for resources: a biocontrol agent might reduce the benefits conferred by a desirable animal species by consuming a plant on which the animal prefers to lay its eggs. Another example of indirect negative consequences occurred in England when a virus introduced to control rabbits reduced the amount of open ground (because large rabbit populations reduce the ground cover), in turn reducing underground ant nests and triggering the extinction of a blue butterfly that had depended on the nests to shelter its offspring. The paucity of known extinctions or disruptions resulting from indirect interactions may reflect not the infrequency of such mishaps but rather the failure to look for or to detect them: most organisms likely to be adversely affected by indirect interactions are of little or no known commercial value and the events linking a biocontrol agent with an adverse effect are often unclear. Moreover, determining the potential risks of biocontrol agents before they are used is difficult, especially when a nonnative agent is introduced, because, unlike a chemical pesticide, a biocontrol agent may adapt in unpredictable ways so that it can feed on or otherwise harm new hosts.

According to the passage, which of the following is a

concern that arises with biocontrol agents but not with chemical pesticides?


    (A) Biocontrol agents are likely to destroy desirable species as well as undesirable ones.

    (B) Biocontrol agents are likely to have indirect as well as direct adverse effects on nontarget species.

    (C) Biocontrol agents may change in unforeseen ways and thus be able to damage new hosts.

    (D) Biocontrol agents may be ineffective in destroying targeted species.

    (E) Biocontrol agents may be effective for only a short period of time. 


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答案:
C
定位比较困
难,基本定位在全文中间的部分。 (A)生物控制有可能在毁灭想要消灭的物种的同时也消灭了不想要消灭的物种。从文中的第一句可以看出,生物控制的好处就在于可以消灭特定的物种。也就是说化学制剂可能会消灭掉其它不应该被消灭的物种。后文有提到说,生物控制也可能会对其它物种产生影响。所以,这点不是两者的不同点。 (B)生物控制有可能用直接的和非直接的影响在非目标生物上。从文中可以看出,生物控制不能直接影响非目标生物。第一句就阐明了生物控制不会影响其它生物,但是后文阐述了会有非直接的影响。 (C)Correct. 生物控制可能会有不可预见性的改变并且有能力毁掉新的物种。这个选项定位在“especially when a nonnative agent is introduced, because, unlike a chemical pesticide, a biocontrol agent may adapt in unpredictable ways so that it can feed on or otherwise harm new hosts.”。几乎是原文的直译。 (D)生物控制也许会对想要消灭的物种无效。文中没有讨论消灭目标物种的有效性问题。 (E)生物控制也许只在一段时间内有效。文中没有提到生物控制是否长期有效。

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