The old man was seen in the park at 7:00 a.m. yesterday.A. exerciseB. exercisedC. to exer

问题补充：

The old man was seen in the park at 7:00 a.m. yesterday.

A. exercise

B. exercised

C. to exercise

D. exercising

答案：

A. exercise

B. exercised

C. to exercise

(答案→)D. exercising

解析：在主动语态中，see sb doing/see sb do ，本题是被动语态，表示在昨天的7点，正在进行的动作，故选D