信息系统主要是指由（）组成的人机系统。
A.计算机硬件
B.计算机软件
C.网络和通信设备
D.信息资源
E.信息用户
参考答案：A, B, C, D, E
由权值为1,2,3,4,5,6的六个叶子结点构成一棵哈夫曼树，则带权的路径的长度为______。
Things went well for her during her early life but in her middle age her ______ seemed to change.
A．affair
B．luck
C．event
D．chance
