单选题 From the passage we know that ______.
   A．virus scanners can identify a virus even if they don't have a signature for it
   B．one of the weaknesses of the virus scanners is that they can't detect a known virus before it is executed
   C．self mutating viruses which infect a file in a different way each time, can't be identified by a simple pattern search and therefore make the scanners ineffective
   D．because virus scanners identify a virus by name, they can indicate only where on the hard driver infection has occurred