9.It is uncertain side effect the medicine will bring about,

9．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 答案：B
解析：本题考查主语从句的引导词.
根据句意：尽管大约有两千名病人服用过这种药物,但是,它会带来什么样的副作用还不确定.
side effect意思是“副作用”,还原主语从句中的主干是the medicine will bring about ______ side effect.
故选择what,形容词“什么样的”,起修饰作用.