单项选择题

When carbon is added to iron in proper ______ the result is steel.

A．rates
B．thicknesses
C．proportions
D．densities
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单项选择题
Doctors warned against chewing tobacco as a ______ for smoking.
A．relief
B．revival
C．substitute
D．succession
单项选择题
Joe is not good at sports, but when it ______ mathematics, he is the best in the class.
A．comes to
B．comes up to
C．comes on to
D．comes around to
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