如图为验证动量守恒定律的实验装置，实验中选取两个半径相同、质量不等的小球，按下面步骤进行实验：

①用天平测出两个小球的质量分别为m₁和m₂；

②安装实验装置，将斜槽AB固定在桌边，使槽的末端切线水平，再将一斜面BC连接在斜槽末端；

③先不放小球m₂，让小球m₁从斜槽顶端A处由静止释放，标记小球在斜面上的落点位置P；

④将小球m₂放在斜槽末端B处，仍让小球m₁从斜槽顶端A处由静止释放，两球发生碰撞，分别标记小球m₁、m₂在斜面上的落点位置；

⑤用毫米刻度尺测出各落点位置到斜槽末端B的距离．图中从M、P、N点是实验过程中记下的小球在斜面上的三个落点位置，从M、P、N到B点的距离分别为S_M、S_P、S_N．依据上述实验步骤，请回答下面问题：

根据短文理解，回答问题

    Many students in the USA always take the school bus to school every day. Parents think taking the school bus is very safe. But sometimes it is also dangerous (危险的). Some students die (死亡) because they don't follow the school bus rules. Here are some school bus rules:

◆Arrive at the bus station early.

◆Wait for the bus at the bus station.

◆Don't play around (在周围) the bus.

◆Don't run everywhere at the bus station.

◆Don't walk or run in the bus when it is going.

◆Don't try to leave the bus before it stops.

These rules are good for students. Remember them and follow them. So you can keep yourself safe.

日积月累。