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More selective thanmost chemical pesticides in that they ordinarily destroy only unwanted species,biocontrol agents (such as insects, fungi, and viruses) eat, infect, orparasitize 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 desirableanimal species by consuming a plant on which the animal prefers to lay itseggs. Another example of indirect negative consequences occurred in Englandwhen a virus introduced to control rabbits reduced the amount of open ground(because large rabbit populations reduce the ground cover), in turn reducingunderground ant nests and triggering the extinction of a blue butterfly thathad depended on the nests to shelter its offspring. The paucity of known extinctions ordisruptions resulting from indirect interactions may reflect not theinfrequency of such mishaps but rather the failure to look for or to detectthem: most organisms likely to beadversely affected by indirect interactions are of little or no knowncommercial value and the events linking a biocontrol agent with an adverseeffect are often unclear. Moreover,determining the potential risks of biocontrol agents before they are used isdifficult, especially when a nonnative agent is introduced, because, unlike achemical pesticide, a biocontrol agent may adapt in unpredictable ways so thatit can feed on or otherwise harm new hosts.
Question #6. 091-03 (21514-!-item-!-188;#058&000091-03)
According to thepassage, which of the following is a concern that arises with biocontrol agentsbut not with chemical pesticides?
(A) Biocontrolagents are likely to destroy desirable species as well as undesirable ones.
(B) Biocontrolagents are likely to have indirect as well as direct adverse effects onnontarget species.
(C) Biocontrolagents may change in unforeseen ways and thus be able to damage new hosts.
(D) Biocontrolagents may be ineffective in destroying targeted species.
(E) Biocontrolagents may be effective for only a short period of time.
请教A为什么不对？ 我觉得A也可以。。。。做过的大侠们帮帮忙。。
另外这一句：
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. 请问第一句中的they是指chemical pesticides 还是control agents？ |
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发表于 2011-8-6 09:26:10
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