单项选择题

Please_________from smoking until the aeroplane is airborne.

A．refrain
B．prevent
C．resist
D．restrain
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单项选择题
He _______ in Japan as he left by boat last month.
A．ought to arrive
B．ought to have arrived
C．should arrive
D．must arrive
单项选择题
______is the center of our planetary system was a difficult concept to grasp in the Middle Ages.
A．It is the sun and not the earth
B．Being the sun and not the earth
C．The sun and not the earth
D．That the sun and not the earth
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