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  3. The economy in the United States is heavily dependent on aluminum, a material widely used in the construction of buildings and i

The economy in the United States is heavily dependent on aluminum, a material widely used in the construction of buildings and i

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问题     The economy in the United States is heavily dependent on aluminum, a material widely used in the construction of buildings and in making such diverse things as cars, airplanes, and food containers. In 1979 Americans used over five million tons of new aluminum, and one and a half million tons of recycled aluminum. Some ninety percent of the bauxite(矾土)ore from which new aluminum is normally derived had to be imported to meet the demand. Poorer ores are abundant in the United States, however, researchers at Purdue University may recently have found a way to obtain aluminum magnetically from these.
    Although aluminum is not attracted by ordinary magnets, under special conditions it becomes temporarily "paramagnetic" , or very weakly responsive to a magnetic field. This is achieved by immersing ore particles in water to which certain salts have been added and then filtering the ore through steel wool in the presence of a strong magnetic field. It is hoped that this technique will reduce the amount of high-grade aluminum the United States must import.
By immersing ore particles in water, ______.
选项 A、we can get ore particles of equal size
B、we can purify aluminum particles
C、we can filter out aluminum
D、we can improve the working conditions around
答案C
解析 本题的依据是最后一句(It is hoped that this technique will reduce the amount ofhigh—grade aluminum the United States must import．)。显然，把矿粒浸入水中的做法目的只有一个，即提取铝。故选C。
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