阅读理解 The Grand Canyon Standing on the rim of the Grand Canyon(大峡谷), gazing across this giant wound in the Earth''s surface, a visitor might assume that the canyon had been caused by some ancient convulsion(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 1,000 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 actions upwarding (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 upward, 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 carreside canyons. Pushing slowly backward into the plateau, the side canyons expose new rocks, and the pattern of erosion continues.

单选题 The passage mainly discusses______.

【正确答案】 B

【答案解析】第一段指出大峡谷的形成是由于缓慢有序的侵蚀形成的；第二段又说几百万年前大峡谷一带还是平面的，后来被逐渐侵蚀；第三段说由于地面隆起，便有了高原河流，也有了著名的大峡谷；最后一段又进一步说明了大峡谷的进一步形成是由于力的作用。因而本篇讨论的是形成大峡谷的各种力。故选B)。

单选题 The word "wound" (Line 1 ,Para. 1 ) indicates that______.

单选题 According to the passage, how many miles long was the trench that became known as the Grand Canyon?

单选题 It can be concluded from this passage about the Grand Canyon that______.

【正确答案】 A

【答案解析】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)。