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