单项选择题

At the present rate，approximately how many years later the black top soil now covering the Midwest will completely be blown or washed away？（）

A.120 years later.
B.80 years later.
C.50 years later.
D.100 years later.

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单项选择题
According to the selection，streams overtop their banks partly because（）.

A.material from higher land is washed into them
B.ditches and drains lead into them
C.rivers become too swift
D.snow melts more rapidly nowadays
单项选择题
All of the following cause floods EXCEPT（）.

A.heavy rain
B.melting snow
C.increasing storm runoff
D.porous soil
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