单项选择题
The output volume of a positive fixed displacement pump can be changed only by （）.
A.changing the angle of the tilting plate B.changing the speed of the pump C.moving the slide block and rotor D.moving the shaft trunnion block
单项选择题 An excessive charge of refrigerant in an air-cooled refrigeration system can cause （）.
单项选择题 When the steering gear is in operation, you should （）.
单项选择题 The oil in a cargo winch gear box should be sampled periodically to （）.
