单项选择题

The passage focuses on ______.

A．construction and manufacturing industry in the United States
B．new technique of obtaining aluminum from poor ores
C．consumption and production of aluminum in the United States
D．the annual import of high-grade aluminum into the United States

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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

单项选择题

It is believed that a new technique to ______ will probably be found.

A．recycle old aluminum products
B．discover more bauxite ores
C．make aluminum by putting ore particles under water
D．take out aluminum magnetically from poorer ores