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.

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单项选择题
It can be inferred from the passage that during the past fifty years the demand for magnesium has（）

A．declined greatly
B．remained stable
C．increased slightly
D．risen dramatically
单项选择题
According to the passage, why is magnesium important to industry（）

A．It is strong.
B．It conducts heat well.
C．It weighs little.
D．It is inexpensive to produce.
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