单项选择题
【案例分析题】
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 1979Americans 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，and 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.
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