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

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

admin2016-11-13  82
问题     Our knowledge of the oceans hundreds of 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 (挖掘) 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 (地形). 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 averages about 1 in 30, but 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.
Before the 19th century probably no one thought
选项 A、the sea was two-dimensional.
B、the sea was dangerous
C、the sea floor must be flat.
D、the sea floor was irregular.
答案D
解析 根据题干的Before the 19th century定位到第l段。该段开头就指出，几百年前人们就认为海洋是二维平面；航行在近陆浅水区是不安全的；基本上人人都以为海底是平坦的。由此可见，A、B、C皆与题干不符，可排除。同时，第1段最后指出，19世纪后期研究人员发现海洋的水下特征差异极大，海底并不平坦，这显然不是19世纪前人们的看法，故答案为D。本题采用了逆向思维的考查方式，要求考生辨认出人们没有认识到的内容。考生在解答此类题目的时候可以从问题的相反方向人手，即寻找肯定的表述，然后与选项进行比对，这样便可以很快排除干扰项，得出正确答案。
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