题目内容
（请给出正确答案）
[主观题]
"Do you hear someone knocking at the door？""Yes, I did. I heard him ______ three times."A.
"Do you hear someone knocking at the door？"
"Yes, I did. I heard him ______ three times."
A. knocking
B. knocked
C. being knocking
D. knock
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