单项选择题

Questionsll-14 are based on the following passage. When are airplanes not fuel efficient

A．On short trips.
B．On long trips.
C．When flying over cities.
D．When flying at high altitudes.
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单项选择题
According to the passage, what is one of the reasons why families are getting smaller in American
A．The needs for workers who are able to move at any time have been increasing.
B．Both old and young people prefer to live separately.
C．Children tend to leave their parents when they grow up.
D．People cease to care for having children.
单项选择题
How long did it take for the world’s population to increase from 2 billion to 3 billion
A．304 years.
B．123 years.
C．33 years.
D．14 years.
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