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.
单选题
The passage focuses on______.
【正确答案】
C
【答案解析】解析：本题考查的是对短文主旨大意的理解。首段说的是美国对铝的需求及其来源，第2段讲述了一种新的提取铝的方法。故C．consumption and production of aluminum in the United States最符合题意，A，B，D虽然提到与短文有关的内容，但都不够全面。
单选题
It is believed that a new technique to______will probably be found.
【正确答案】
D
【答案解析】解析：本题的依据是第1段最后一句，关键部分是Poorer ores，have found a way to obtain aluminum magnetically。
单选题
In the second paragraph the word "paramagnetic" means______.
【正确答案】
A
【答案解析】解析：这是个词汇题。解这种题需要先找到该词所在的句子，答案往往从中可以读出。paramagnetic在第2段首句，句子后半部分的or very weakly responsive to a magnetic field就是解释，故选A．slightly magnetic。
单选题
By immersing ore particles in water, ______.
【正确答案】
C
【答案解析】解析：本题的依据是最后一句(It is hoped that this technique will reduce the amount of high-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。
单选题
We understand from the passage that the United States______.
【正确答案】
A
【答案解析】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…)。