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
Indeed, almost every scientist now finds it impossible to read all the works relevant to his own subject, ______ extensively outside of it.
A．much more to read
B．much less to read
C．much less reading
D．still more reading
单项选择题
The supermarket is ______ until 9:30 P. M.
A．opening
B．opened
C．open
D．opens
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