单项选择题

According to the passage, magnesium was first obtained from （）

A．rocks found on land
B．great amounts of ocean water
C．the sea floor
D．major industrial sites

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单项选择题

According to the passage, which of the following was a direct consequence of the new method of obtaining magnesium（）

A．The development of insulation materials.
B．Increased airplane production.
C．Improved medical facilities.
D．The development of the cheap inks for printing.

单项选择题

What is the main topic of this passage（）

A．Use of sea water.
B．Treatment of sea water.
C．Chemical properties of magnesium.
D．Derivation and users of magnesium.