单项选择题
A.Closing of the solenoid valve
B.Closing of the expansion valve
C.An increase in the suction pressure
D.Decreasing the suction pressure
单项选择题 In an electro-hydraulic steering gear system, when will the variable displacement pump be placed on stroke?（）
单项选择题 An additive used to improve the ability of a lube oil to reduce friction is known as a/an （）.
单项选择题 The suction - force principle of operation is a typical characteristic of （）.