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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单项选择题
A．actingB．relyingC．centeringD．commenting
A．acting
B．relying
C．centering
D．commenting
单项选择题
A．beforeB．unlessC．untilD．because
A．before
B．unless
C．until
D．because
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