R－12 is generally considered to be a safe, nontoxic, nonflammable, and non－explosive refri

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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