单项选择题

When our university laboratory bought this microscope we were given a one year's ______.

A. reservation
B. security
C. proof
D. guarantee
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单项选择题
The reporters exposed the corruption of several senior officers in the government; consequently, these officers _________ to resign from office.
A. have asked
B. were asked
C. had asked
D. had been asked
单项选择题
Acute hearing helps most animals sense the approach of thunderstorms long before people __________.

A. do
B. hear
C. do them
D. hearing it
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