【题目】（2016·浙江）Scientists have advanced many theories about why human beings cry tears, none of ________has been proved.
A. whom B. which
C. what D. that
在线课程【答案】B
【解析】试题句意：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。