单项选择题

You cannot master a foreign language ______ you work hard at it.

A．unless
B．when
C．if
D．so
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单项选择题
There are forty students in our class. ______ of them are League members.
A．No one
B．Both
C．All
D．Neither
单项选择题
Seldom ______ any mistake during his past ten years of service there.
A．did he make
B．he makes
C．he made
D．he did make
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