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.
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.
(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.
答案选d,偶用排除法也选了d.
可是这篇文章作者对biocontrol agents的态度难道不是nagetive的么?
只有第一句是一个正评,第二句就来了一个however开头的大转折.整个后面都在讲biocontrol agents的副作用, 难道不是负评么?
汗....文章的感情倾向都搞不明白了~~~ | 
