单选题 Standing on the rim of the Grand Canyon, gazing across this giant wound in the Earth's surface, a visitor might assume that the canyon had been caused by some ancient convulsion. In fact, the events that produced the canyon, far from being sudden and cataclysmic, simply add up to the slow and orderly process of erosion.
Many millions of years ago the Colorado Plateau in the Grand Canyon area contained 10,000 more feet of rock than it does today and was relatively level. The additional material consisted of some 14 layered formations of rock. In the Grand Canyon-region these layers were largely worn away over the course of millions of years.
Approximately 65 million years ago the plateau's flat surface in the Grand Canyon area bulged upward from internal pressure; geologists refer to this bulging action as upwarping; it was followed by a general elevation of the whole Colorado Plateau, a process that is still going on. As the plateau gradually rose, shallow rivers that meandered across it began to run more swiftly and cut more definite courses. One of these rivers, located east of the upwarp, was the ancestor of the Colorado. Another river system, called the Hualapai, flowing west of the upwarp, extended itself eastward by cutting back into the upwarp; it eventually connected with the ancient Colorado and captured its waters. The new river then began to carve out the 277-mile-long trench that eventually became the Grand Canyon. Geologists estimate that this initial cutting action began no earlier than 10 million years ago.
Since then, the canyon forming has been cumulative. To the corrosive force of the river itself have been added other factors. Heat and cold, rain and snow, along with the varying resistance of the rocks, increase the opportunities for erosion. The canyon walls crumble; the river acquires a cutting tool, tons of debris; rainfall running off the high plateau creates feeder streams that carve side canyons. Pushing slowly backward into the plateau, the side canyons expose new rocks, and the pat- tern of erosion continues.

单选题 What does the text 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.

单选题 In line 1, the author refers to the Grand Canyon as a "wound" to indicate that ______.
A. it was caused by some ancient convulsion
B. its presence is an embarrassment to the state of Colorado
C. it looks like an injury on the Earth's surface
D. it has caused many visitors to injure themselves

单选题 According to the text, 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

单选题 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 Hualapai River and what is now the Colorado River.
D. Between the Hualapai River and what is now the Colorado River.

单选题 We can conclude from the text that ______.
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.