单项选择题

Never（）seen so many people cooperating in such an event.

A. is the world
B. has the world
C. the world is
D. the world has

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单项选择题
The leaders met to discuss the problems facing their（）countries.

A. respective
B. respected
C. respectable
D. respectful
单项选择题
Professor Liu（）for 20 years by the end of next summer.

A. has taught
B. will he teaching
C. has been teaching
D. will have taught
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