题目内容
【题目】One morning I was sitting at my desk when a policeman ______ at the door.
A．knocked B．was knocking C．had knocked D．knocks
【答案】A
【解析】句意：一天早上我正坐在桌旁，这时一个警察来敲门。本句是be doing...when句型，when后的从句用一般过去时。故选A。
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题目内容
【题目】One morning I was sitting at my desk when a policeman ______ at the door.
A．knocked B．was knocking C．had knocked D．knocks
【答案】A
【解析】句意：一天早上我正坐在桌旁，这时一个警察来敲门。本句是be doing...when句型，when后的从句用一般过去时。故选A。
