单项选择题
A.water-tube natural circulation boilers
B.fire-tube boilers
C.water-tube forced circulation boilers
D.all of the above
单项选择题 When the tilting box of a variable stroke axial-piston pump is perpendicular to the pump shaft, which of the following conditions will exist?（）
单项选择题 The hydraulic oil most likely to thin out when hot and thick when cold would have a viscosity index of（）
单项选择题 When the steering wheel is turned, oil is directed to the steering gear rams by（）