It is uncertain side effect the medicine will bring about , though about one thousand patients have taken it.
A. that
B. whether
C. what
D. how
网友回答
A. that
B. whether
(答案→)C. what
D. how
解析：
试题分析:句意：尽管大约一千名病人已经服用了这种药物，但是还不能确定这种药会产生什么副作用。根据结构可知it是形式主语，真正的主语是 side effect the medicine will bring about ,主语从句中缺少bring about 的宾语，应该用连接代词，故答案选C。