单项选择题

What steps could be taken to improve the air

A．Reduce the amount of nitrogen.
B．Increase the release of oxygen.
C．Reduce nitrous oxide emissions.
D．Stop industrial activities.
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单项选择题
Luis Posada Carriles was detained________.
A．because he had killed 73 Cuban civilians in 1976
B．because he had planned an airliner bombing in 1976
C．because he had worked as a spy on Cuba’’s behalf
D．because of his illegal immigration status
单项选择题
According to the passage, when are children first expected to study hard
A．Before 6 years of age.
B．Between 6 and 10.
C．After 10 years of age.
D．After 12 years of age.
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