斜面长10米,高2米,机械效率80%,将一重为400牛的物体沿斜面底端匀速拉到顶端,则物体与斜面之间的摩擦力为( )
—Would you like to have dinner with us tonight?
—___________
被誉为“中华水塔”的三江源地区,位于我国的( )