单项选择题

—Are you sure ______ you've met him before

A．unless
B．that
C．when
D．though
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单项选择题
-- I really don't know how to thank you enough.--______.
A．No problem
B．Think nothing of it
C．Not at all
D．It doesn't matter
单项选择题
How to raise the efficiency, ______ is the question.
A．what
B．which
C．that
D．if
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