We were having a lesson a policeman came in.A. when B. whileC. until D. before

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【题目】We were having a lesson 　　　　 a policeman came in.

A. when B. while

C. until D. before

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【答案】A

【解析】考查固定句式。句意：我们正在上课，突然一个警察进来了。此处when用作并列连词，表示＂就在那时＂，相当于and at that time。故选A。