【单选题】

WASHINGTON -- considering the serious warnings lately about Internet viruses(病毒), users may don’t use their machines just to keep them safe. In reality, common sense and a few basic tips can keep Internet surfers secure.

Anti-virus software is a must, and it should be updated at least once a week to recognize newly viruses. If this is not done, you will meet great troubles.
People with broadband Internet connections should have also a firewall. These protect the computer from attacks by hackers(黑客).
Then it is time to think about computer habits. Unlike old viruses, which spread only through shared disks, the Internet brings viruses straight to the e-mail box. ff an e-mail with attachment(附件) comes from an unknown sender, or unexpectedly from a known’ sender, it should not be openeD、
On receiving an unexpected attachment from a friend, experts suggest a telephone call to make sure of the mail’ s source before opening. If you open the mail without any consideration, you will be easily attacked by the hackers.
Why should anti-virus software be updated at least once a week
A、Because the newly published anti-virus software is more expensive.
Because the previous(先前的) anti-virus software cannot work any longer.
C．Because the inventors of viruses have suggested this ide
A、
D．Because computer viruses are always developing.