单项选择题
A.maintain a preset pressure on the line
B.closely regulate the amount of flow
C.relieve excessive pressure on the line
D.allow flow in one direction only
单项选择题 Sluggish response or action of the hydraulic actuators may be a result of （）.
单项选择题 A knocking sound from one cylinder of an operating air compressor indicates （）.
单项选择题 Rotation of the steering wheel on the navigation bridge initiates oil pressure being applied to the steering gear rams by （）.