[单项选择]DoS攻击的目的是（）
A. 获取合法用户的口令和账号
B. 使计算机和网络无法提供正常的服务
C. 远程控制别人的计算机
D. 监听网络上传输的所有信息

网络安全包含了网络信息的可用性、保密性、完整性和真实性。防范Dos攻击是提高 (1) 的措施，数字签名是保证 (2) 的措施。

DOs and DON’Ts
The following DOs and DON’Ts may help you to avoid becoming the victim of a pickpocket.
DO carry with you only what you need; leave in your hotel’s safe such valuables as jewelry, airline tickets, travelers checks, credit cards, extra cash and your passport. Once admitted to France, American tourists do not have to carry passports at all times, a student card or driver’s license usually is enough if you are asked by a police to officer for identification.
DO carry your passport separately from your wallet or handbag~ This is not always feasible for women, but men can carry their passports in a front pants pocket or inside jacket pocket.
DON’T have one family or group member carry everyone’s passport; never put a group’s passports in the same bag or briefcase.
DON’T leave your valuables in a locked car, locks are easily opened by experts, the trunk is not safe either.
DOs