阅读成都武侯祠内的一副对联，回答问题。

两表酬三顾，一对足千秋。

对联中“两表”指的是____ ，“一对”指的是____ 。

增加：在缺词处加一个漏字符号（∧），并在其下面写出该加的词。

删除：把多余的词斜线（＼）划掉。

修改：在错的词下划一横线，并在该词下面写出修改后的词。

注意：1）每处错误及修改均仅限一词；

2）只允许修改10处，多者（从第11处起）不计分。

    Yesterday morning, I heard a piece of news which my friend Li Lei was admitted to the hospital because of the car accident. The day before yesterday, Li Lei is going to a bookstore to buy a book. He was very carefully when he got to the crossroads. He crossed the street when the traffic light turned to green. However, if he just went to the middle of the street, a car suddenly appeared at his right-hand side and came directly towards him. The driver drove fast and it was too late to avoiding it. Li Lei was knocked by the car. She was carried to the hospital immediately and had a surgery. Last night, I visited him. He looked pale unless he was out of danger.

若三个有理数的和为0，则下列结论正确的是（　　）

为了乘客的安全，北京地铁1号线在距离车体一定的位置设置了一条黄色的安全线。请你用相关的物理知识解释为什么要设置安全线。