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? | 
