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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.
We understand from the passage that the United States______.
选项 A、is rich in low-grade aluminum-bearing ores
B、is short of aluminum-making techniques
C、has to import ninety percent of its annual need of aluminum
D、has to produce fifty percent of the world’s supply to meet its annual aluminum demand
答案A
解析 这是一个细节题，答案依据是第1段第4句(Poorer ores are abundant in the United States…)。
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