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单选题
R-12 is generally considered to be a safe, nontoxic, nonflammable, and non-explosive refrigerantIt can, however, become highly toxic when （）.
A
superheated outside the system
B
in contact with an open flame
C
heated to the boiling point
D
All of the above
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考题
单选题为什么自1993年禁止使用R-12？（）A
禁止使用R-12是因为其易燃B
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