单项选择题

______ is the center of our planetary system was a difficult concept to grasp in the Middle Age.

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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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
单项选择题
Too much ______ to X-rays can cause skin burns, cancer or other damage to the body.
A．disclosure
B．exhibition
C．contact
D．exposure
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