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
网友回答
A. that
(答案→)B. what
C. how
D. whether
解析：考查宾语从句。宾语从句中 bring about后缺少宾语，故用what 作定语修饰side effect，句意：还不清楚这种药会带来什么样的副作用，虽然大约两千名病人已经服用过。故选B。