在网上进行信息交流及商务活动时，需要通过（）来证明各实体的电子身份。
A.数字证书
B.数字签名
C.公共密钥
D.私有密钥
参考答案：A
_____ for his action on the playing field ,but he is also famous for the things that he does off the playing field.
A．Not he is only known
B．Not only is he known
C．Only is he known not
D．He is only not known
下列选项中属于维护管理的是（）。
A.安全防范
B.转移使用
C.维护计划
D.消防管理
