She looks very sad. I want to help her but I just don’t know to do it.A. whatB. whenC. h

问题补充：

She looks very sad. I want to help her, but I just don’t know to do it.

A. what

B. when

C. how

D. which

答案：

A. what

B. when

(答案→)C. how

D. which

解析：疑问词+to do sth是固定搭配，充当某个句子成分。在此句中充当句子的宾语。