问题补充:
I was almost asleep last night when I suddenly heard someone at the door.
A. be knocking
B. knocking
C. to knock
D. having knocked
答案:
A. be knocking
(答案→)B. knocking
C. to knock
D. having knocked
解析:本题考查对动词习惯用法的掌握。 hear sb. doing sth.意为听见某人正在做某事。