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  3. Standing on the rim of the Grand Canyon(大峡谷), gazing across this giant would in the Earth’s surface, a visitor might assume that

Standing on the rim of the Grand Canyon(大峡谷), gazing across this giant would in the Earth’s surface, a visitor might assume that

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问题    Standing on the rim of the Grand Canyon(大峡谷), gazing across this giant would in the Earth’s surface, a visitor might assume that the canyon had been caused by some ancient convulsing (震动). In fact, the events that produced the canyon, far from being sudden and catastrophic, 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 1000 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 actions upwading(弯曲上升); 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 winded across it began to run more swiftly and cut more definite courses. One of these rivers, located east of the up- ward, was the ancestor of the Colorado. Another river system, called the Hualapai, flowing west of the upward, extended itself eastward by cutting back into the upward, 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 erosions, 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 pattern of erosion continues.
It can be concluded from this passage about the Grand Canyon that ______.
选项 A、its shapes 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
答案A
解析 最后一段“从那以后，大峡谷的形成日积月累，除了河流的侵蚀作用外，还有其他因素，如冷热、雨雪以及岩石的各种阻力增加了侵蚀的可能性。大峡谷壁面塌落，河流获得了切割工具。成吨的碎石和从高原上流下的雨水产生的支流切割峡谷壁，又逐步侵蚀到高原，峡谷边暴露出新的岩石，而侵蚀仍在继续”。可见应选[A]。
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