I often saw the boysin the river.A. swamB. swimmingC. to swimD. swim

问题补充：

I often saw the boysin the river.

A. swam

B. swimming

C. to swim

D. swim

答案：

A. swam

B. swimming

C. to swim

(答案→)D. swim

解析：句意：我经常看到这些男孩子在河里游泳。see sb. do sth.看到某人做某事，看到了全过程；see sb. doing sth.看到某人正在做某事，看到正在进行的动作，故选D。