单选题：根据以下材料，回答题。HackingPeople tend to think of computers as isol

根据以下材料，回答题。
Hacking
People tend to think of computers as isolated machines，working away all by themselves.Some do--personal computer without an outside link，like someone’S hideaway(隐蔽的)cabin in the woods.But just as most of homes are tied to a community by streets，bus routes and electric lines，computers that exchange intelligence are part of a community--local，national and even global network joined by telephone connections.
The computer network is a creation of the electric age.but it is based on old—fashioned trust.It cannot work without trust.A rogue loose(为所欲为的无赖)in a computer system called hac-ker(黑客)is worse than a thief entering your house.He could go through anyone’s electronic mail or add to.change，distort or delete anything in the information stored in the computer’s menlory.He could even take control of the entire system by placing his own instructions in the software tl_lat runs it. He could shut the computer down whenever he wished，and no one could stop him.Then he could program the computer to erase any sign of his ever having been there·
Hacking.our electronic—age term for computer break—in，is more and more in the news-brainv kids vandalizing university records，even pranking(胡闹)about in supposedly safeguarded svstems.To those who understand how computer networks are increasingly regulating life in the late 20th century.these are not laughing matters.A potential for disaster is buildin9：A dissatisfied fonner insurance—company employee wipes out information from payroll(工资表)files.A student sends out a“virus"，a secret and destructive command，over a national network.The virus copies itself at lightning speed，jamming the entire network--thousands of academic，eommercial and government computer systems.Such disastrous cases have already occurred.Now exists tlle Dossibility of terrorism by computer. Spoiling a system responsible for air—traffic control at a busy airport.or knocking out the telephones of a major city，is a relatively easy way to spread Danic.Yet neitller business nor government has done enough to toughen its defenses against attack.For one thin9.such defenses are expensive;for another，they may interrupt communieation-tlle main reason for using computers in the first place.
The writer mentions“a thief”in the second paragraph . A．to show that a hacker is more dangerous than a thief
B．to tell people that thieves like to steal computers nowadays
C．to demand that a protective computer system should be set up against thieves
D．to demonstrate that hackers and thieves are the same people