__________great fun they are having in the sitting room!
A:What
B:How
C:What a
D:How a
物体A静止在水平面上,请画出物体A所受力的示意图.
如图所示,用手竖直捏住物理课本,手和课本都在空中静止。下列各对力中属于平衡力的是( )