单选题
In Bt cotton fields, some pests replaced the bollworm as the primary pest because ______.
A、
their favorite food is Bt cotton
B、
the bollworms were killed out by the Bt cotton
C、
both their enemies and the bollworm were all killed by Bt cotton
D、
people used the pesticides
【正确答案】
C
【答案解析】
第四段中说到，GM棉花对棉铃虫的寄生性天敌有害。可以推断，GM棉花不仅杀死了棉铃虫，还同时杀死了棉铃虫的天敌，于是本来处于次要位置的害虫自然就在数量上占了优势，成为主要害虫。
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