It is uncertain ______ side effect the medicine will bring about,although about two thousand patient

题目

It is uncertain ______ side effect the medicine will bring about,although about two thousand patients have taken it.
A.that
B.what
C.how
D.whether
这题选的是B,如果改成It is uncertain ______ the medicine will bring about side effect...能不能选D?为什么?

答案

如果改的话,可以选D. 因为单看 the medicine will bring about side effect这个句子,是一个有完整句子成分的句子.所以用whether 连接则没有语法错误.