单项选择题
A.Natural convection only
B.Forced convection only
C.Both natural convection and forced convection
D.None
单项选择题 Auxiliary boilers can be classified as（）
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（）