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

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

According to this passage, the standardized measurements were first used ______.

A．when the steel beam was developed
B．when the Egyptians built the pyramids
C．when scientists wanted to make specific observation
D．when engineers designed microscopic measuring devices