When he saw a little boy in the river he took off his shirt and jumped into the water.A.

问题补充：

When he saw a little boy in the river, he took off his shirt and jumped into the water.

A. struggles

B. struggling

C. to struggle

D. struggle

答案：

A. struggles

(答案→)B. struggling

C. to struggle

D. struggle

解析： see sb doing sth “看见某人正在做某事”句意为“当看到小男孩在河水中挣扎时，他脱掉衣服跳进水中。”所以选B。