问题
选择题
I _________ my homework last night when I heard a loud knock on the door.[ ]
A. did
B. was doing
C. have done
D. had done
答案
答案：B
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I _________ my homework last night when I heard a loud knock on the door.[ ]
A. did
B. was doing
C. have done
D. had done
答案：B
