“Did you hear someone knocking at the door?”
“Yes, I did. I heard him______three times.”
A. knocking B. knocked C. being knocking D. knock
“Did you hear someone knocking at the door?”
“Yes, I did. I heard him______three times.”
A. knocking B. knocked C. being knocking D. knock
D
hear sb. do sth. 强调全过程， hear sb. doing sth强调正在做某事。“我听见他敲了三遍了”故排除A.