问题
单项选择题
Since many insects have already become（）to DDT, scientists have to look for some new insecticide
A.active
B.resistant
C.tough
D.gradual
答案
参考答案：A
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Since many insects have already become（）to DDT, scientists have to look for some new insecticide
A.active
B.resistant
C.tough
D.gradual
参考答案：A