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
A．selectionsB．separationsC．seriesD．sections
A．selections
B．separations
C．series
D．sections
单项选择题
A．stableB．splendidC．smoothD．complicated
A．stable
B．splendid
C．smooth
D．complicated
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