单项选择题
A.acid formation
B.sludge formation
C.ice in the expansion valve
D.All of the above
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? （）
单项选择题 The function of the hydraulic telemotor transmitter used in an electro-hydraulic steering gear system is to （）.