已知双曲线$\frac{x^2}{a^2}-\frac{y^2}{b^2}=1\left(a>0,b>0\right)$的右焦点为F(2，0)，设A，B为双曲线上关于原点对称的两点，AF的中点为M，BF的中点为N，若原点O在以线段MN为直径的圆上，直线AB的斜率为$\frac{3\sqrt{7}}{7}$，则双曲线的离心率为（    ）

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.