单项选择题
A.stored at temperatures below 60 degrees Fahrenheit
B.charged into a system having copper fittings
C.allowed to mix with compressor oil
D.exposed to an open flame or hot surface
单项选择题 A refrigeration system contaminated with moisture can be affected by （）.
单项选择题 R-12 is generally considered to be a safe, nontoxic, nonflammable, and non-explosive refrigerantIt can, however, become highly toxic when （）.
单项选择题 Which of the following statements is/are true regarding hydraulic pumps in general? （）