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 （）.
A.superheated outside the system B.in contact with an open flame C.heated to the boiling point D.All of the above
单项选择题 Which of the following statements is/are true regarding hydraulic pumps in general? （）
单项选择题 The function of the hydraulic telemotor transmitter used in an electro-hydraulic steering gear system is to （）.
单项选择题 To obtain maximum efficiency, two stage air compressors are usually （）.
