问题补充：
----What were you doing this time yesterday?
----I my homework at John’s house.
A. do
B. am doing
C. did
D. was doing
答案：
A. do
B. am doing
C. did
(答案→)D. was doing
解析：句意：昨天这个时候你在做什么？我正在约翰的家里做作业。句中有昨天的这个时候，所以句子要用过去进行时态的形式，表示过去的某个时间正在做某事，故选D
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