Among the basic categories of equipment, which of the following are used to convert the hydraulic energy into linear motion?（）

A.the slewing motor
B.the luffing motor
C.the hoisting motor
D.none of the above

A.unusual vibration
B.excessive worn parts
C.abnormal pressure
D.any of the above

A.Relief valve
B.Direction control valve
C.Stop valve
D.Pressure control valve