He has become a millionaire and owns a big computer company. He is really _________ great success in _________ business. A. a; 不填 B. the; the C. a; the D. 不填;不填 答案:A一个20kg的小孩从高度为3.0m的滑梯顶端由静止开始滑下,滑到底端时的速度大小为2.0m/s.取g=10m/s2,关于力对小孩做的功,以下结果正确的是( )A.合力做功40JB.阻力做功-640JC.重力做功600JD.阻力做功-560J