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24-Passage one
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.
(Line 14)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 of 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 (35) 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.
我对文章的理解：
生物载体可以用来消灭目标植物和有害动物。（+）
然而，可能会对非目标物种带来负面的影响。（-）---T.S, 举例：
1， 与非目标物种竞争资源。如：生物载体通过吃掉有益物种用来产卵的植物从而侵占了这些物种的利益。
2， 产生间接的负面结果。大量兔子数量会造成地面覆盖减小，当用来控制兔子数量的病毒被引进来降低无覆盖的地面时，反过来会降低地面下的蚁巢，引发依赖这种蚁巢庇护后代的蓝蝴蝶的灭亡。由于间接作用导致的少量灭绝可能反映了不仅仅是少量的灾祸，而且是难以寻找和检测到他们：受间接作用相反影响的大多数生物是很少的或者不了解其商业价值，并且这种反作用并不清楚。另外，在生物载体被使用前决定他们的潜在危险是困难的，尤其当一个非本地的载体被引进，因为，不象化学杀虫剂，生物载体可以以不可预料的方式适应环境，以至于能够猎食或者伤害新的主生物体。
24-4 The passage suggests that the author would be most likely to agree with which of the following statements about the use of biocontrol agents?
(A) Biocontrol agent should be used only in cases where chemical pesticides have proven ineffective or overly dangerous.---not mention
(B) Extinctions and disruptions resulting from the use of biocontrol agents are likely to have increasingly severe commercial consequences.
(C) The use of biocontrol agents does not require regulation as stringent as that required by the use of chemical pesticides.
(D) The use of biocontrol agents may even finally supersede the use of chemical pesticides in controlling unwanted species
(E) The risks of using native biocontrol agents may be easier to predict than the risks of using nonnative biocontrol agents.---unlike chemical pesticides,nonnative is unpredictable.---没有与native agents相比。
answer: E.
我选D.
因为文章第一句说明了biocontrol agent与chemical petiscides的比较关系。more selective表示biocontrol agent更加具有选择性,=supersede.
虽然文章最后一句提到了nonnative biocontrol agent ...unpredictable ways, 但是正如文章所说，unlike chemical petiscides,这是与chemical petiscides相比.因此不能得出与native agent比较关系。
请指点。
24-6 The example presented by the author in lines 14-23 most clearly serves to illustrate
(A) a situation in which a species is less vulnerable to biocontrol agents than it would have been to chemical pesticides.
(B) a way in which the introduction of a biocontrol agent can affect a nontarget species.
(C) a nonnative agent’s adapting in an unpredictable way that results in damage to a new host.
(D) the contention that biocontrol agents can harm nontarget species by competing with them for resources.
(E) the way in which indirect consequences from the use of biocontrol agents are most likely to occurs.
answer :B
my choice :E
我的理解：文章的结构正如上面中文显示，中心内容是说明生物载体会产生负面影响。然后举了两个例子，分别说明不同方面的负面影响。L14-23的例子是要说明产生的间接副作用这一方面的。而B直接回答了文章的T.S.
请指教。 | 
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发表于 2005-6-16 16:43:00
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