【题目】Scientists have advanced many theories about why human beings cry tears , none of ______ has been proved.
A.whom
B.which
C.what
D.that

【答案】B
【解析】考查定语从句。句意：关于人类为什么哭的时候会流泪，科学家已经提出了很多理论，没有一个被证明了。使用定语从句，先行词是theories，指物，定语从句中用代词+介词+关系代词which引导。故选B。