WASHINGTON—Considering　the　serious　warnings　lately　about　Internet　viruses　(病毒),　users　may　be　tempted　to　unplug　their　machines　just　to　keep　them　safe.　In　reality,　common　sense　and　a　few　basic　tips　can　keep　Internet　surfers　secure.Antivirus　software　is　a　must,　and　it　should　be　updated　at　least　once　a　week　to　recog-nize　newly　released　viruses.People　with　broadband　Internet　connections　should　also　have　a　firewall.　These　pro-tect　the　computer　from　attacks　by　hackers　(黑客).Then　it　is　time　to　think　about　computer　habits.　Unlike　old　viruses,　which　spread　on　ly　through　shared　diskettes,　the　Internet　brings　viruses　straight　to　the　emai-box.　if　email　with　attachment　comes　from　an　unknown　sender,　or　unexpectedly　from　a　known　sender,　it　should　not　be　opened.On　receipt　of　an　unexpected　attachment　from　a　friend,　experts　suggest　a　telephone　　call　to　make　sure　of　the　mail?s　source　before　opening.Why　should　antivirus　software　be　updated　at　least　once　a　week?　Because　______.

A.
the newly published antivirus software is more expensive

B.
the previous antivirus software cannot work longer than a week

C.
the inventors of viruses have suggested this idea

D.
computer viruses are always developing

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