Once harm to the environment, it takes years to have the system recovered.A．doesB．is doneC．will be done D．be done 题目和参考答案——青夏教育精英家教网——

题目内容

Once harm ______ to the environment, it takes years to have the system recovered.

A．does

B．is done

C．will be done

D．be done

B

解析

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