She looks very sad. I want to help her but I just don’t knowDto do.A. what B. when C. ho

问题补充：

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

A. what B. when C. how D. where

答案：

解析：“疑问词 + 动词不定式”是英语中常用的一种结构。在句中可以作主语、动词或介词的宾语以及表语。