I was almost asleep last night when I suddenly heard someone at the door.A. be knockingB.

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。