单项选择题
A.in motion, providing a null input
B.not in motion, thus providing a null input
C.in motion, providing an input to place the main pump on maximum stroke
D.in motion, providing an input to place the main pump at null stroke
单项选择题 In a fixed displacement axial piston hydraulic motor, the speed is varied by（）
单项选择题 In a shell-and-tube type hydraulic fluid cooler, the amount of heat transferred from the hydraulic fluid to the cooling water depends upon（）
单项选择题 An orifice-check valve placed in a hydraulic system is used to（）