单项选择题
External and internal leakage in hydraulic systems may be caused by （）.
A.unusual vibration B.excessive worn parts C.abnormal pressure D.any of the above
单项选择题 What type of valve is used to direct the flow of hydraulic fluid?（）
单项选择题 The rotor of the hydraulic motor rotates at a speed proportional to the （）.
单项选择题 （） are used to limit the pressure in any particular portion and mainly control the pressure after the valve.
