液压系统动作不规则的原因有()。
A.油中混有空气
B.指令信号不稳定
C.阀芯卡涩
D.缸或马达磨损或损坏
E.放大器失灵或调得不对
参考答案:A, B, C, D, E
四边形的内角和等于( );四边形的外角和等于( )。
With trees and grass__________39.6% of the urban area,the city has taken on a new look.
A.covering
B.covered
C.to be covered
D.being covered