Since teenagers have ____ ____these needs, they often question who they are and how they ____ ____ society.
下列关于我国现行行政区划的叙述,正确的是( )
如图所示,用滑轮组拉动重为70N的物体A,使物体A在水平方向上移动5m,所用拉力F为20N,地面对物体A的摩擦力为30N,则拉力F做的功为____ J,滑轮组的机械效率为____ .