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：句意：尽管大约1000名病人使用了这种药，但它会带来什么样的副作用还不清楚。此处what修饰名词side effect。故选C。