单项选择题

(That) the sun (but) not the earth (is) the center of our planetary system (was) a difficult concept to grasp in the Middle Ages.

A．That
B．but
C．is
D．was
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单项选择题
The new instrument is (far) superior (than) the old (one) we bought three years (ago).
A．far
B．than
C．one
D．ago
单项选择题
(Though) some people have suggested that women (would return to) housework (in order to) leave more jobs for men, but the idea has been rejected (by) both women and men in public polls.
A．Though
B．would return to
C．in order to
D．by
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