伺服阀分为（）、（）、（）三大类。.
参考答案：电液；气液；机液
22. —Has the boy got his bicycle now?
23. —Yes, the police gave .
24.
A．him to him
B．it to it
C．it to him
D．him to it
由硬质颗粒或摩擦表面划伤以及表层材料（）的现象称为磨粒磨损。
A、脱离
B、脱皮
C、脱落
D、剥落
