阅读下列短文，从每题所给的四个选项（A、B、C和D）中，选出最佳选项。
A
In today's world, we rely on computers as never before．They are used for everything from ordering a pizza to running hospitals and military defense systems．Banking and credit card information is stored and accessed by computers．So what happens when a computer gets infected with a virus? One effect is that people's access to their e-mail accounts is cut off．A more serious possible consequence is that billions of dollars could be lost．
A virus is a computer program that copies itself onto other programs and infects them． Similar to an easily spread disease, a computer virus goes from computer to computer, either adding to or changing the tasks a program is designed to do．
The first computer viruses were created in the mid '80s and had varying effects．Some caused files to be deleted, or made the letters on the screen appear to fall off．Others displayed a specific message once the computer was turned on．
Viruses today are much more widespread and dangerous than ever
