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  3. Magnesium is another mineral we now obtain by collecting huge volumes of ocean water and treating it with chemicals. Although or

Magnesium is another mineral we now obtain by collecting huge volumes of ocean water and treating it with chemicals. Although or

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问题     Magnesium is another mineral we now obtain by collecting huge volumes of ocean water and treating it with chemicals. Although originally it was derived only from brines or from the treatment of such magnesium-containing rocks as dolomite, of which whole mountain ranges are composed. In a cubic mile of sea water there are about four million tons of magnesium. Since the direct extraction method was developed about 1941, production has increased enormously. It was magnesium from sea that made possible the wartime growth of the aviation industry, for every, air plane made in the United States (and in most other countries as well) contains about haft a ton of magnesium metal. And it has innumerable uses in other industries where a lightweight metal is desired, besides its long-standing utility as an insulating material, and its use in printing inks, medicines, and toothpaste.
It can be inferred from the passage that during the past fifty years the demand for magnesium has
选项 A、declined greatly
B、remained stable
C、increased slightly
D、risen dramatically
答案D
解析
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