单项选择题
In a general cargo crane, （） is fitted to raise or lower the jib.
A.the slewing motor B.the luffing motor C.the hoisting motor D.none of the above
单项选择题 External and internal leakage in hydraulic systems may be caused by （）.
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 （）.
