mrs smith looks very busy we went to help her but we don't knoe__求大神详细讲解 和求详细讲解选项

Mrs Smith looks very busy.We want to help her,but we don’t know _______.
A.what to do it B.how to do C.what to do D.why to do
是这道题吧
选C
D不论是意思还是用法都错了
首先意思……我们不知道为什么要做（前面说了因为看见别人忙要帮助,但这里却说不知道为什么.自相矛盾）
其次,一般why的语法知识
why not do sth=why don't you do sth.为什么不做某事呢?