题目

Many teenagers（青少年） feel that the most important people in their lives are their friends. They believe that their family members don’t know them as well as their friends do. In large families, it is quite often for brothers and sisters to fight with each other and then they can only go to their friends for some ideas. It is very important for teenagers to have one good friend or a group of friends. Even when they are not with their friends, they usually spend a lot of time talking among themselves on the phone. This communication is very important in children’s growing up, becau若代数式x2-6x+b可化为（x-a）2-1，则b-a的值（　　）A．3B．4C．5D．6