单项选择题

The fifth generation computers, with artificial intelligence, ______ and perfected now.

A. developed


B. have developed
C. are being developed

D. will have been developed
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单项选择题
In the southwestern part of the United States ______ built in the last century.
A. there are many abandoned mining towns
B. were many abandoned mining towns
C. where there are many abandoned mining towns
D. where are many abandoned mining towns
单项选择题
All flights ______ because of the snowstorm, many passengers could do nothing but take the train.
A. had been cancelled

B. have been cancelled
C. were cancelled


D. having been cancelled
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