阅读下面的短文，掌握其大意，然后从短文后各题所给的A、B、C、D四个选项中选出可以填入相应空白处的最佳答案。

    Every day some people are killed while they are crossing the roads. 1 of these people are old people and children.Old people are often killed because they can't see or hear very well.Children are killed because they are not 2 .They forget to look and listen before they 3 the roads.

    A car or a bus can't stop quickly.If a car is going very fast, it will travel many meters before it stops.Some people don't always understand this.They think a car can stop in a few meters.It is difficult to 4 how fast a car is moving.The only way to cross the roads safely is to look both ways, right and left.Then if the roads are 5 , you can cross them.

A、B两列简谐横波均沿x轴正向传播，在某时刻的波形分别如图甲、乙所示，经过时间t（t小于A波的周期T_A），这两列简谐横波的波形分别变为图丙、丁所示，则A、B两列波的波速v_A、v_B之比不可能的是（   ）

甲数的2倍比乙数大3，甲数的3倍比乙数的2倍小1，若设甲数为x，乙数为y，则根据题意可列出的方程组为（　　）

请你帮助数学兴趣小组的同学们共同解决如下问题．

研究问题农民李伯伯在一个新鱼池中养了两种鱼：鲫鱼、鲤鱼，怎样估算鲫鱼、鲤鱼的数量？

操作方法先从鱼池中任意捕捞出100条鱼，做上标记后放回鱼池中，一个星期后再次捕捞．

活动结束再次任意捕捞出80条鱼，统计结果如右图．

根据上述的摸球实验，请你帮助李伯伯估算：