如图，用“碰撞实验器”可以动量守恒定律，即研究两个小球在轨道水平部分碰撞前后的动量关系．

a若入射小球质量为m₁，半径为r₁；被碰小球质量为m₂，半径为r₂，则____

①m₁＞m₂，r₁＞r₂

②m₁＞m₂，r₁＜r₂

③m₁＞m₂，r₁=r₂

④m₁＜m₂，r₁=r₂

b为完成此实验，需要使用的测量工具除了刻度尺外还必需的是____．

c安装轨道时，轨道末端必须____．

d设入射小球的质量为m₁，被碰小球的质量为m₂，P为碰前入射小球落点的平均位置，则关系式____（用m₁、m₂及图中字母表示）成立，即表示碰撞中动量守恒．

    I have a small room. There is a bed, a desk and two chairs in it. There are some books and a pencil-box on the desk. There is a photo(照片)on the wall. It's a photo of my family. There is a map of China and two pictures on the wall, too. My room is nice. I like it very much.