单项选择题

We find that some birds ______twice a year between hot and cold countries.

A．transfer
B．migrate
C．commute
D．mobilize
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单项选择题
It is well known that knowledge is the ______condition for expansion of mind.
A．incompatible
B．incredible
C．indefinite
D．indispensable
单项选择题
In spring, traffic was often______ along the roads to holiday places.
A．added up
B．built up
C．held up
D．pulled up
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