All employees are required to ensure that the computers are turned off before______the office.

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问题 All employees are required to ensure that the computers are turned off before______the office.

选项 A、leaving
B、has left
C、to leave
D、leave

答案A

解析语法：动名词1．before可作为介词，连词使用。2．before做连词时，后面应接完整的句子，(B)选项和(D)选项没有主语，因此排除。3．before做介词时，后不能接不定式，因此(C)选项排除．动名词(A)为正确选项。

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