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  3. Secret codes (密码) keep messages private. Banks, companies, and government agencies use secret codes in doing business, especiall

Secret codes (密码) keep messages private. Banks, companies, and government agencies use secret codes in doing business, especiall

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问题     Secret codes (密码) keep messages private. Banks, companies, and government agencies use secret codes in doing business, especially when information is sent by computer.
    People have used secret codes for thousands of years.【R1】________. Code breaking never lags (落后) far behind code making. The science of creating and reading coded messages is called cryptography.
    There are three main types of cryptography.【R2】________. For example, the first letters of "My elephant eats too many eels" spell out the hidden message "Meet me."
    【R3】________. You might represent each letter with a number, For example. Let’s number the letters of the alphabet, in order, from I to 26. If we substitute a number for each letter, the message "Meet me" would read "13 5 20 13 5."
    A code uses symbols to replace words, phrases, or sentences. To read the message of a real code, you must have a code book.【R4】________. For example, "bridge" might stand for "meet" and "out" might stand for "me." The message "bridge out" would actually mean "Meet me."【R5】________. However, it is also hard to keep a code book secret for long. So codes must be changed frequently.
    A. It is very hard to break a code without the code book.
    B. In any language, some letters are used more than others.
    C. Only people who know the keyword can read the message.
    D. As long as there have been codes, people have tried to break them.
    E. You can hide a message by having the first letters of each word spell it out.
    F. With a code book, you might write down words that would stand for other words.
    G. Another way to hide a message is to use symbols to stand for specific letters of the alphabet.
【R3】
选项
答案G
解析 主旨句。上段指出有三种主要的密码设定方式，上段已经提到了一种，本段应该指出另一种。下文举例说明了用数字来代替字母的密码方式，由此可知G项能起到统领本段的作用，意为“另一种隐藏信息的方式就是用符合来代替字母表中特定的字母。”故选G。
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