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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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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、禁止使用R-12是因为CFC很难大规模生产C、禁止使用R-12是因为CFC破坏臭氧层D、禁止使用R-12是因为需要采用大量的臭氧来制造R-12
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单选题下列有关空调冷媒R134a和R-12比较叙述，那项有误？（）A
臭氧层的破坏指数，R134a较低B
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R134a较R-12易吸收水分
考题
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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
禁止使用R-12是因为CFC很难大规模生产C
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