单项选择题

The passage mainly discusses______.

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 passage would most likely be found in a texbook on which of the following subjects

A．Astronomy.
B．Botany(植物学).
C．Geology.
D．Chemistry.

单项选择题

The reader can conclude that______.

A．countries of the world will work together on pollution problems
B．some industries are now making economic use of liquid wastes
C．by-products from wastes lead to a more prosperous marketplace
D．science is making great progress in increasing water supplies