现代密码体制使用的基本方法仍然是替换和(52)。

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The main function of a processor is to(68)and carry out instructions which usually consist of operation and(69). The term to measure the speed of the processor is called a Megahertz(MHz), which means million(70)per second.

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The successful spread of a virus depends on how long it can()unnoticed, before its presence is made known by the activation of side-effects.

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Anti- virus software protects your computer from viruses by(75)your computer's memory and disk devices.

(70)is not a peripheral component or device. The basic input device on a small computer is a(71).

A(74)is a device that enables the computer to handle sounds.

The transmission media used t0 build a LAN is(73).