I ______ housework when there was a knock at the door.

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问题 I ______ housework when there was a knock at the door.

选项 A、did
B、was doing
C、am doing

答案B

解析 本题考查动词时态。when引导的时间状语从句是一般过去时,主句也应用过去时,义因为主句动词do是延续性动词,所以主句时态应用过去进行时。
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