问题
单项选择题
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
参考答案：B
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Scientists have advanced many theories about why human beings cry tears , none of （） has been proved.
A. whom
B. which
C. what
D. that
参考答案：B