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Magnesium is anther 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
5 a cubic mile of seawater there are abut four million tons of magnesium.
Since the direct extraction method was developed about 1941, production
has increased enormously. It was magnesium from the sea that made
possible the wartime growth of the aviation industry, for every airplane
made in the United States (and in most other countries as well) contains
10 about half 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 toothpastes.
1. What is the main topic of this passage?
2. According to the passage, magnesium was first obtained from
3. According to the passage, which of the following was a direct consequence of the new method of obtaining magnesium?
4. According to the passage, why is magnesium important to industry?
5. It can be inferred from the passage that during the past fifty years the demand for magnesium has
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