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  3. Our knowledge of the oceans a hundred years ago was confined to the two-dimensional shape of the sea surface and the hazards of

Our knowledge of the oceans a hundred years ago was confined to the two-dimensional shape of the sea surface and the hazards of

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问题     Our knowledge of the oceans a hundred years ago was confined to the two-dimensional shape of the sea surface and the hazards of navigation presented by the irregularities in depth of the shallow water close to the land. The open sea was deep and mysterious, and anyone who gave more than a passing thought to the bottom confines of the oceans probably assumed that the sea bed was flat. Sir James Clark Ross had obtained a sounding of over 2,400 fathoms in 1839, but it was not until 1869, when H. M. S. Porcupine was put at the disposal of the Royal Society for several cruises, that a series of deep soundings was obtained in the Atlantic and the first samples were collected by dredging the bottom. Shortly after this the famous H. M. S. Challenger expedition established the study of the sea floor as a subject worthy of the most qualified physicists and geologists. A burst of activity associated with the laying of submarine cables soon confirmed the Challenger’s observation that many parts of the ocean were two to three miles deep, and the existence of underwater features of considerable magnitude.
    Today, enough soundings are available to enable a relief map of the Atlantic to be drawn and we know something of the great variety of the sea bed’s topography. Since the sea covers the greater part of the earth’s surface, it is quite reasonable to regard the sea floor as the basic form of the crust of the earth, with, superimposed upon it, the continents, together with the islands and other features of the oceans. The continents form rugged tablelands which stand nearly three miles above the floor of the open ocean. From the shore line, out a distance which may be anywhere from a few miles to a few hundred miles, runs the gentle slope of the continental shelf, geologically part of the continents. The real dividing line between continents and oceans occurs at the foot of a steeper slope.
    This continental slope usually starts at a place somewhere near the 100-fathom mark and in the course of a few hundred miles reaches the true ocean floor at 2,500-3,500 fathoms. The slope contains steep, probably vertical, cliffs, and gentle sediment-covered terraces, and near its lower reaches there is a long tailing-off which is almost certainly the result of material transported out to deep water after being eroded from the continental masses.  
The continent slope________.
选项 A、gradually stretches out to the sea bed
B、starts immediately at the seashore
C、ends at the place near the 100-fathom mark
D、contains steep and vertical cliffs
答案A
解析 事实细节题。最后一段第一句指出，大陆坡一般是从差不多100英寻的地方开始，一直延伸到几百英里远深达2500到3500英寻的真正海底，即大陆坡是逐渐延伸至海底，A项表述正确，故为答案，同时排除B项和C项。最后一段第二句讲到，大陆坡包括陡峭的乃至垂直的峭壁和沉积物覆盖的缓和的阶梯地带，在这个地带的低处是很长的一段尾沙地带。D项表述与原文不相符，故排除。
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