题目内容：

Choose the best answer and complete the passage （选择最恰当的选项完成短文）

Secret codes 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. As long as there have been codes, people have tried to break them. Code breaking never ______ code making. The science of creating and reading coded messages is called cryptography（密码系统）.

There are three main types of cryptography. You can ______ a message by having the first letters of each word spell it out. For example, the first letters of “My elephant eats too many eels（鳗）” spell out the message “Meet me.”

Another way is to use symbols to ______ specific letters of the alphabet. 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 replace each letter with a number, the message “Meet me” would read “13, 5, 5, ______, 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. With a code book, you might write down words that would be ______ as other words. For example, “bridge” might refer to “meet” and “out” might mean “me.” The message “Bridge out” would have meaning of “Meet me”. It is very hard to break a code with out the code book. Actually, it is also hard to keep a code book secret for long. ______, codes must be changed frequently.

1.A.goes ahead B.takes the place of C.falls behind D.depends on

2.A.send B.take C.translate D.hide

3.A.stand for B.make up C.deal with D.find out

4.A.17 B.18 C.19 D.20

5.A.explained B.misunderstood C.remembered D.acted

6.A.However B.Therefore C.In addition D.Otherwise