-- Where are the trees? -- They are beside the r____.
已知正数x,y满足x+2y=1,则1x+1y的最小值为( )
如图所示,把带负电荷的橡胶棒靠近悬挂的轻质小球时,发现轻质小球被推开,说明轻质小球带____ 电荷,还说明力可以改变物体的____ .