Since many insects have already become ____ to DDT, scientists have to look for some new insecticide. ( 章节：6 难度：3)
- A、active
- B、resistant
- C、tough
- D、gradual
正确答案:B
答案解析:有
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Since many insects have already become ____ to DDT, scientists have to look for some new insecticide. ( 章节：6 难度：3)
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