单项选择题

A number of theories have been proposed to explain the situation.

A. suggested
B. tested
C. used
D. announced
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单项选择题
Everybody was glad to see Mary back.()

A. sorry
B. sad
C. angry
D. happy
单项选择题
There are various ways to solve the problem.
A. same
B. many
C. helpful
D. different
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