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.

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单项选择题

According to the passage, the first phenomenon to contribute to 'the formation of the Grand Canyon was ______.

A．a series of volcanic eruptions
B．the collapse of rock formations in the Colorado Plateau
C．a succession of floods from the Hualapai River and what is now the Colorado River
D．the earth's internal pressure lifting the Colorado Plateau region

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.