This morning when I walked by I saw many children in the park.A. playedB. playC. playin

问题补充：

This morning, when I walked by, I saw many children in the park.

A. played

B. play

C. playing

D. to play

答案：

A. played

B. play

(答案→)C. playing

D. to play

解析：本题的含义是今天早晨当我路过的时候，我看到许多孩子在公园玩，对于我在走当时，看到孩子们应该正在玩，see sb doing sth表示看某人正在做某事，故选C。