防火墙是隔离内部和外部网的一类安全系统。通常防火墙中使用的技术有过滤和代理两种。路由器可以根据(47)进行过滤，以阻挡某些非法访问。(48)是一种代理协议，使用该协议的代理服务器是一种(49)网关。另外一种代理服务器使用(50)技术，它可以把内部网络中的某些私有IP地址隐藏起来。安全机制是实现安全服务的技术手段，一种安全机制可以提供多种安全服务，而一种安全服务也可采用多种安全机制。加密机制不能提供的安全服务是(51)。

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When you open a file or run a program in a computer, a window appears on the desktop of your computer. The(67)of the window indicates which program you are currently working in arid its color will become blue if a window is active. When you click on The(68), the window disappears and appears as a button on the taskbar. Clicking on a button in(69)will invoke a specific command.

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A(66)holds enough bits to store a single character while a(67)is a group of bytes.

The HTML is the language used to create(73)for use on the WWW.

Voice-(72)technology converts human speech into a digital code that a computer can under- stand.

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