单项选择题
A.misalignment or worn main bearings
B.insufficient cylinder lubrication
C.defective or poorly fitted valves
D.all of the above
单项选择题 Why are removable sleeves installed on centrifugal pump shafts?（）
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 （）.