A woman was waiting for her plane at an airport.

    She bought a bag of cookies，found a place to 1and took out her book.She started to read and2noticed that a man，sitting beside her，took a cookie from her bag.She just 3not to see this.So she ate the cookies quickly and at the same time watched how the shameless thief was stealing her cookies.She thought to herself，“If I wasn't such a(n)4 person，I would hit him in the eye.”

    When only one cookie was left，she looked with interest and 5what he would do.He smiled，took the 6cookie and broke it in half.She thought，“This guy is so impolite，he didn't even show any thanks”.

    After her flight was called，the woman 7her luggage and headed to the gate.She took her seat in the 8and started to look for her book，which was almost completed.As she reached her luggage，she gasped(喘气)with 9 ， because there was an unopened bag of cookies.A terrible thought crossed her mind，“If my cookie bag is here，the other one was his and he just tried to 10”. She realized that she was the impolite one，but it was too late to say sorry to him.

A. It is very hard to break a code without the code book.

B. Only people who know the keyword can read the message.

C. As long as there have been codes, people have tried to break them.

D. You can hide a message by having the first letters of each word spell it out.

E. With a code book, you might write down words that would stand for other words.

F. Another way to hide a message is to use symbols to stand for specific letters of the alphabet.

    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. {#blank#}1{#/blank#}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. {#blank#}2{#/blank#}For example, the first letters of "My elephant eats too many eels" Spell out the hidden message "Meet me."

    {#blank#}3{#/blank#}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, 15, 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. {#blank#}4{#/blank#} For example, "bridge" might stand for "meet" and "out" might stand for "me." The message "bridge out" would actually mean "Meet me." {#blank#}5{#/blank#} However, it is also hard to keep a code book secret for long. So codes must be changed frequently.