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  3. Under the world’s oceans and seas lies a world of lands that stretches for hundreds of kilometers in every direction. In this wo

Under the world’s oceans and seas lies a world of lands that stretches for hundreds of kilometers in every direction. In this wo

admin2010-09-12  92
问题     Under the world’s oceans and seas lies a world of lands that stretches for hundreds of kilometers in every direction. In this world without a sun there are deep canyons(峡谷), great plains, and towering ranges of mountains. And this underwater world covers 70 percent of the earth’s surface.
    The world’s longest mountain range extends 16, 000 kilometers under the Atlantic Ocean from Iceland to the Antarctic. Called the Mid-Atlantic ridge, this range of mountains contains a number of high peaks(山峰), among which are the Ascension Islands, the Azores, and Iceland.
    The world’s highest mountain is found in the mid-Pacific area. It is the Hawaiian volcano Mauna Kea, which is 9, 450 meters from base to peak. This is nearly 610 meters taller than Mount Everest. Only 4, 221 meters of Mauna Kea rise above the surface to the sea. In 1873, the British survey ship Challenger discovered the deepest trench(沟)in all the seas. Named the Challenger Deep, it is near the island of Guam and is 11 kilometers deep.
    Since sunlight only reaches to a depth of about 244 meters, many of the creatures of the deep live in a world of darkness. However, to aid them in their survival, many are equipped with their own lighting system. Food comes to them from the upper levels of the ocean and the waste matter that gradually moves downward toward the ocean floor assures a constant supply. These undersea creatures must remain at various levels to which they are adapted. To stray(偏离、走失)to other levels could result in death from improper water pressure.
The undersea creatures have to remain at their own level of the sea because______.
选项 A、some creatures are not equipped with their own lighting system
B、different levels are of different water pressure
C、the water temperature varies from level to level
D、different levels are of different brightness
答案B
解析
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