It began as a : high school and college students studying computer technology figured out they could use personal computers to break into telephone company computers and make free, long-distance telephone calls. These young computer gifts soon gained the nickname 'hackers （黑客）'. Police arrested a few hackers, but many went on to even more complex . One hacker was arrested for illegal telephone calls and later used a prison phone to change a police officer's credit records to get back at （报复） the officer for arresting him. The hacker also used a computer to change his college records to give himself better grades. As hackers gained experience, they began invading computers at banks, airlines and other businesses. In one plan, a hacker instructed an airline's computer to give him free airplane tickets. The U.S. government is worried hackers may break into its complex networks of defense computers. The government's classified secrets are vulnerable （脆弱的） because thousands of government computers are connected by telephone lines that hackers can tap into. In November 1988, a college student tapped into a non-classified U.S. Defense Department computer network called Arpanet. The hacker injected （输入，注入） a computer program that left copies of itself throughout Arpanet. Some hackers use each 'virus ' to destroy all the data in a computer. But in this case, government officials shut down the network before the program reached every computer in the system. Shutting down the system angered many researchers who were using the computers. The hacker turned himself in （自首） to police and told them how to get his program out of the computer system. He was charged with a crime. The incident attracted computer in the United States. Many companies have hired experts to protect their computers from hackers, and many computer experts now advise companies on how to protect their computers. The U.S. government believes foreign governments have hired hackers to try to break into top-secret defense computers. It fears a hacker could inject a virus into military computers that would clean up all the data during a war. Experts disagree over whether a computer network can ever be safe from . But in the future, some of the most brilliant minds in the U.S. will be working to ruin the efforts of computer hackers and spies. 71．The main idea of the article is that _______________． A．computer hackers only want to make free long-distance phone calls B．the government wants to hire computer hackers to spy on the Russians C．computer hackers are a er to private companies and government secrets D．many companies have hired experts to protect their computers from hackers carrying viruses 72．A computer 'virus' is like a human virus because it _______________． A．makes a computer cough and throw up B．spreads from one computer to another C．can clean up a person's doctor bills D．requires regular visits to a doctor 73．A hacker can be erous because ________________． A．he knows how to make free long-distance telephone calls B．information is stored in computers, and a hacker knows how to destroy the information C．once in prison, he can use a telephone to operate his computer D．a hacker who steals a free airplane ticket might take a doctor of that seat on the airplane 74．U.S．government computers are vulnerable to a hacker because _____________． A．the government always pays its telephone bills on time B．the Russians know what's in the U.S．computers C．viruses attack only government computers D．many government computers are connected by telephone lines 75．In the future ________________． A．some most brilliant minds in America will be working to stop computer hackers and spies B．hackers will begin invading computers at banks, airlines and other businesses C．many computer hackers will be arrested for free telephone calls D．some computer hackers will turn themselves in to the police