简述液压伺服系统的特点。
参考答案：
液压伺服系统具有以下特点：1.随动；2.放大；3.反馈；4.偏差。
The letters for the boss ______ on his desk but he didn’t read them until three days later.
A．was put
B．put
C．has put
D．were put
大面积龋损修复时，固位钉在修复体与牙本质中长度一般应为（）
A.小于2mm
B.大于2mm
C.2mm
D.1～1.5mm
E.1mm
