单项选择题

Air is composed of______ gas including hydrogen, oxygen and carbon dioxide.

A．varied
B．variant
C．variable
D．various
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单项选择题
In order to repair barns, build fences, grow crops, and care for animals a farmer must indeed be_______.
A．restless
B．skilled
C．strong
D．versatile
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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