一质点做直线运动,从静止开始匀加速,紧接着匀减速直到停止,总共花时间3s,已知这3个1s内质点的位移之比为4:11:5,则质点达到最大速度的时刻是( )
A:85s
B:45s
C:53s
D:73s
-15℃比15℃低30度
I am pleased____ (help) you.
He that he knew nothing about computer, so he began to work hard at it.
如果把绿叶比作“绿色工厂”,那么其动力是( )