单项选择题

By immersing ore particles in water, ______.

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

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单项选择题

In the second paragraph the word paramagnetic means ______.

A．slightly magnetic
B．a weak magnetic field
C．a strong magnetic substance
D．the under-water magnetic power

单项选择题

We understand from the passage that the United States ______.

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