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单项选择题

Top soil ______.

A．is good for agriculture
B．is useless and unproductive
C．is found only in desert regions
D．is dry and sandy
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单项选择题According to the passage some desert areas were once ______.

A．fertile lands
B．covered by rivers
C．very cold
D．very dry and useless lands.

单项选择题If there were no trees ______.

A．the land would became fertile
B．heavy rains would clean the tropics
C．the rich top soil couldn't be held
D．there would be no agriculture