单项选择题

What do people usually think contributed to the decline of North Africa

A．Human factors.
B．Economic factors.
C．Climatic factors.
D．Population factors.
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单项选择题
The 15 plates of the Earth are formed from ______.
A．the oceanic crusts and continental crests
B．the crusts and the mantle
C．the crusts and the top and solid part of the mantle
D．the continental crusts and the solid part of the mantle
单项选择题
Which of the following is the ultimate reason for the decline of tropical rain forests
A．Unfavorable climate.
B．Human abuse of forests.
C．Increase in the temperature.
D．Unusual rainfalls.
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