As the mid- examination is coming, the students are busy ____ it. A. preparing B. prepare for C. prepare D. preparing for 答案:D有A,B,C,D四个城市,它们各有一个著名的旅游点依此记为a,b,c,d.把A,B,C,D和a,b,c,d分别写成左、右两列,现在一名旅游爱好者随机用4条线把左右全部连接起来,构成“一一对应”,如果某个旅游点是与该旅游点所在的城市相连的(比如A与a相连)就得2分,否则就得0分;则该爱好者得分的数学期望为 .