换一种方式研究高考真题

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 lags far behind code making. The science of creating and reading coded messages is called cryptography.

There are three main types of cryptography. You can hide 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” Spell out the hidden message “Meet me.”

Another way to hide a message is to use symbols to stand for 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 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. With a code book, you might write down words that would stand for other words.For example,”bridge“ might stand for “meet” and “out” might stand for “me.” The message “bridge out” would actually mean “Meet me.” It is very hard to break a code without the code book.However, it is also hard to keep a code book secret for long. So codes must be changed frequently.

A Heroic Driver

Larry works with Transport Drivers. Inc. One morning in 2009, Larry was traveling along 165 north after delivering to one of his customers. Suddenly, he saw a car with its bright lights on. As he got closer, he found another vehicle upside down on the road. One more look and he noticed flames shooting out from under the disabled vehicle. Larry pulled over, set the brake and got hold of the fire extinguisher （灭火器）. Two good bursts from the extinguisher and the fire was put out.

The man who had his bright lights on came over and told Larry he had made an emergency call. They then heard a woman’s voice coming from the wrecked （毁坏的）vehicle. Approaching the vehicle, they saw that a woman was trying to get out of the broken window. They told her to stay still until the emergency personnel arrived, but she thought the car was going to explode. Larry told her that he had already put out the fire and she should not move in case she injured her neck.

Once fire and emergency people arrive, Larry and the other man backed off and let them go to work. Then, Larry asked the police if he was needed or free to go. They let him and the other man go.

One thing is for certain—Larry went above and beyond the call of duty by getting so close to the burning vehicle! His efforts most likely saved the woman’s life.