I saw them football on the playground when I passed by.A. playingB. were playingC. playedD

问题补充：

I saw them football on the playground when I passed by.

A. playing

B. were playing

C. played

D. to play

答案：

(答案→)A. playing

B. were playing

C. played

D. to play

解析：句意：我经过的时候看见他们正在操场上踢足球。See sb. doing sth.看见某人正在做某事，后接动词ing形式，表示动作正在发生，故选A。