题目内容：
阅读部分第17题：By the mid-1960s，cotton farmers in Central America found that pesticidesAwere wiping out 50% of the pests plaguing the crops．
Bwere destroyin9 50% of the crops they were meant to protect．
Cwere causing a 50% increase in the number of new pests reported．
Dwere costin9 50% of the total amount they spent on their crops．
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答案解析：