单项选择题
A.either all the way open or all the way closed
B.normally open
C.throttled depending on load
D.controlled by suction pressure
单项选择题 When a hydraulic vane pump is working, which of the following is in contact with the vanes?（） (1) the rotor (2) the internal surface of the housing (3) the external surface of the housing
单项选择题 Which of the following items is necessary for all waste heat boiler installations, regardless of design or manufacturer? （）
单项选择题 Prior to lighting off a cold automatically fired auxiliary boiler, yon should （）.