斜面长是斜面高的3倍,用50N的力沿此斜面把重100N的物体从斜面底端匀速推到顶端,则斜面的机械效率是 ______.
66.7%
— May I put my pen on the desk?
— _______________.
A. Yes, put it
B. Yes, put them here
C. Yes, please
凿岩台车对设计排位线的误差不得大于()mm。
A.±200
B.±500
C.±1000