单项选择题

Air moves under such conditions that______.

A．there is air pressure
B．the air is heavy
C．air pressure doesn't remain the same all the time
D．air comes up and down
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单项选择题
According to the passage, we can conclude that in some areas if farmers who grow grapes light fires in the early morning, that is because the farmers want to______.
A．drive away warm air
B．prevent cold air from coming to harm their plants
C．cause more wind
D．stop warm air running away
单项选择题
What might be the author's possible purpose in writing this passage
A．To tell readers that horse appeared earlier than human being.
B．To inform us that horses can help us a lot.
C．To introduce the history and characteristics of horses.
D．To introduce the primitive homes to modem man.
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