People are often killed while c____  the road. Most of them are old people and c____ . Old people are often killed because they are careless. People should look and listen b____  they cross the road. A car, a truck or a bus can't stop very q____ if it is going very fast. It will travel many meters before it s____ . Pedestrian(行人)don't know this. They think that a car can stop right away. It is h____ for a pedestrian to know how fast a car is traveling. The only s____ way to cross the road is to look at b____ ways, right and left. The correct way to cross the road is t____  walk quickly when the traffic l____ turn green.

如图所示，在上表面光滑的小车上放有质量m₁的物块和质量为m₂的小球，且m₁＞m₂，开始时物块，小车．小球一起在水平面上匀速运动，现让小车向右匀加速运动则（　　）

一朵花的最主要部分是____ ，种子的最主要部分是____ ．

在中国历史上，分裂和统一的局面曾交替出现。结束东汉末年以来近百年分裂局面，实现全国统一的关键人物是（　　）