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(2016·新课标Ⅰ) 主题：密码的设置 词数：192 (密码)keep messages private.Banks use secret codes in doing business，especially when information is sent by computer. People have used secret codes for thousands of years.__1__ Code breaking never lags(落后)far behind code making.The science of creating and reading coded messages is called cryptography. __2__ For example，the first letters of “My elephant eats too many eels” spell out the hidden message “Meet me.” __3__ You might represent each letter with a number，for example.Let's number the letters of the alphabet，in order，from 1 to 26.If we substitute a number for each letter，the message “Meet me” would read “13 5 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.__4__ For example，“bridge” might stand for “meet” and “out” might stand for “me”．The message “Bridge out” would actually mean “Meet me.” __5__ 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. | 
