单项选择题
A.They make it easier to replace the pump shaft packing
B.They can be economically replaced as they wear out
C.They can be removed when it is necessary to lighten the weight of the pump
D.They increase the strength of the shaft
单项选择题 In reference to air conditioning, when air attains the maximum amount of moisture it can hold at a specific temperature, it is said to be （）.
单项选择题 An axial轴向 piston pump differs from a radial piston pump as the pistons of an axial piston pump are positioned （）.
单项选择题 The usual number of single-acting pistons used in a variable stroke axial-piston pump is （）.