问题
选择题
One evening he heard someone _____ his door. He quickly opened the door and found a stranger. [ ]
A. knocked at
B. knocking at
C. knocking into
答案
答案：B
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One evening he heard someone _____ his door. He quickly opened the door and found a stranger. [ ]
A. knocked at
B. knocking at
C. knocking into
答案：B