90. The sustained massive use of pesticides in farming has two effects that are especially pernicious. First, it often kills off the pests' natural enemies in the area. Second, it often unintentionally gives rise to insecticide-resistant pests, since those insects that survive a particular insecticide will be the ones most resistant to it, and they are the ones left to breed.
From the passage above, it can be properly inferred that the effectiveness
of the sustained massive use of pesticides can be extended by doing which of the following, assuming that each is a realistic possibility?
(A)Using only chemically stable insecticides
(B)Periodically switching the type of insecticide used
(C)Gradually increasing the quantities of pesticides used
(D)Leaving a few fields fallow every year
(E)Breeding higher-yielding varieties of crop plants
90.Choice B gives a way of counteracting a serious drawback of the sustained massive use of pesticides. By periodically changing the pesticide used, pests resistant to one pesticide might be killed by the next pesticide, and those resistant to that pesticide might be killed by another, and so. Therefore, B is the best answer.
Choice A is inappropriate, since the effects of stable pesticides would simply be more persistent. Gradually increasing pesticide amounts (choice C) will likely have no effect on pests already resistant to massive amounts. Leaving a few fields fallow (choice D) is not relevant to the effectiveness of sustained use of pesticides. Breeding higher-yielding crops (choice E) might temporarily increase yields, but not because of anything to do with pesticides.
为什么
(C)Gradually increasing the quantities of pesticides used
不对呢?而且我觉得B只照顾了一个负面效果,而C照顾了两个:
1.减少对益虫的伤害
2.我的理解:从少到多使用,可以避免很快得让害虫产生抗药性 | 
