单项选择题

As ______ known to all, language has been developed through social contacts.

A．is
B．are
C．though
D．if
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单项选择题
The old man has two rooms, one of ______ is used as his study.
A．which
B．that
C．them
D．whom
单项选择题
The heavy rain ______ for several days and all the roads were flooded.
A．went up
B．kept on
C．fell down
D．broke down
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