单项选择题

The author mentions all of the following as causes of erosion EXCEPT ______.

A．wind
B．rain
C．heat
D．cold

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单项选择题

Which of the following conclusions about the Grand Canyon can be drawn from the passage

A．Its contours are constantly changing.
B．It contains approximately 14 million tons of rock.
C．Its eruptions have increased in recent years.
D．It is being eroded by toxic waste and pollutants.

单项选择题

What was the geographic position of the upwarp approximately 65 million years ago

A．To the east of what is now the Colorado River.
B．To the west of the Hualapai River.
C．At the source of the Hualapai River and what is now the Colorado River.
D．Between the Hualapai River and what is now the Colorado River.

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