what 作主语时,谓语动词通常用单数,但是也可以用复数,主要决定于提问者对于 what 所代表的数的理解。例如:
What I say and do are my own affair. / What I say and do is my own affair. (摘自 A Comprehensive Grammar of the English Language by Randolph Quirk et al,1985)
但是,be 动词后面接复数名词时,be 要用复数形式:
What are the constituents of water? 水的构成成分是什么? (《外研社英汉多功能词典》)