题目

  We’ve heard it before---we’ve heard it on the news, from teachers, from parents －children and teenagers today are growing up too fast. There are not too many people that will   13   with that statement. Teenagers are faced with serious problems and decisions at an early   14  . In fact most teenagers’ daily schedules are as   15   as those of an adult’s.   I have been working since I was thirteen, and always in   16   in which I was working with adults. I have had to learn to think and   17   like an adult to be taken seriously. So, I count myself as one当重力对物体做正功时，物体的 [　　] A．重力势能一定增加，速率一定减小 B．重力势能一定减小，速率一定增加 C．重力势能不一定减小，速率一定增加 D．势能一定减小．速率不一定增加