如图所示，PQ为一固定水平放置的光滑细长杆，质量均为m的两小球A、B穿于其上，两球被穿于杆上的轻弹簧相连．在A、B两球上还系有长度为2L的轻线，在轻线中间系有质量不计的光滑定滑轮E，C、D球质量分别为m和2m，用轻绳连接并跨过定滑轮．用手水平托住D球并保持D球静止状态，此时轻弹簧长度也为L，已知劲度系数为K，（弹簧在弹性限度内，重力加速度为g）求：

一切交往的前提是（     ）

    I was only nine years old when I learnt how to use a computer．My mother is my first teacher．I know how to type，how to copy a file，and how to visit a website on the Internet．One day，mom was not at home．I turned on the computer and began to learn how to chat on net．The first one I met there was a boy called Tom．He greeted me politely．When he knew that I was only a 9-year-old girl and almost knew nothing about chatting on net，he started showing me how to use the chatting tools，how to download，and how to send e-mails．I learnt a lot from him．