针对网络系统受到的威胁，OSI安全体系结构提出了几类安全服务，其中不包括（）。
A.访问控制
B.身份验证
C.数据保密
D.别名控制
参考答案：B
阅读理解。
Hello! My name is Alice. I'm from America. I'm twelve years old. My father is a businessman. My
mother is a teacher. Lily is my sister. She is ten years old. She is a student. I am a student, too. We go
to school together, because we are in the same school.
1. Where is Alice from?
A. England
B. the U. S.
C. Canada
2. What is Alice's father?
A. teacher
B. doctor
C. businessman
3. Is Lily a student?
A. Yes.
B. No.
C. I'm not.
4. Is Alice a teacher?
三个等量点电荷位于等边三角形的三个顶点，电性如图所示，A、B是底边中垂线上的两点，C、D两点在底边上，且与中垂线等距。用EC、ED表示C、D两点场强的大小，用φA、φB表示A、B两点的电势，则（ ）
A.EC=ED φA<φB
B.EC<ED φA>φB
C.EC>ED φA>φB
D.EC>ED φA<φB
