单项选择题

What does the passage mainly discuss

A．Patterns of erosion in different mountain ranges.
B．Forces that made the Grand Canyon.
C．The increasing pollution of the Colorado River.
D．The sudden appearance of the Grand Canyon.

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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

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.