OG 90题,大N们帮忙看一下!,谢了!

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

请教大N们: 如果是effectiveness of the sustained massive use 被extended, 为什么是选B) Periodically switching the type of insecticide used, 既然要extendeffectiveness, 为什么不是A) Using only chemically stable insecticides? 如何理解extended?

先谢了!

extend：to make something affect more people, situations, areas etc than before（from longman）

问题的意思是：通过使用下面哪种方法，大规模使用杀虫剂可以得以推广

如果选A的话，就是一直使用同样的杀虫剂，那么对于那些已经对杀虫剂有抵抗力的昆虫而言，杀虫剂对它们无效。

但是若是B的方法，定期的变化杀虫剂的话，那么不同的昆虫对不同的杀虫剂的抵抗能力不同，杀虫剂1没有杀死的昆虫可能被杀虫剂2就可以杀死，这样的话，使用杀虫剂就能起到作用。

Yaxin, Thanks for your detailed explanations !

我在这错误的把effectiveness看成了effect,不过还是词汇功底不够,所以effect指代的是First, it often kills off the pests' natural enemies in the area. Second, it often unintentionally gives rise to insecticide-resistant pests,我就选了A,ETS够高的,不过Yaxin更高!