They seemed each other for a long time.A. knowingB. to knowC. having knownD. to have know

问题补充：

They seemed each other for a long time.

A. knowing

B. to know

C. having known

D. to have known

答案：

A. knowing

B. to know

C. having known

(答案→)D. to have known

解析：考查非谓语动词。句意：他们好像彼此认识好久了。不定式的完成时态表示的是这个动作发生在谓语动作之前。故选D。