Which of the following most logically completes the argument?
A particular computer virus frequently infects personal computers through an email message. Even though virus-scanning software installed on a computer is designed to identify and alarm the user of a virus infection on that computer shortly after it occurs, this particular virus is rarely observed by virus-scanning software. This fact, however, does not mean that the computer's security systems have blocked installation of the virus, because _______________.
【选项】
shortly after it appeared on the Internet, hackers modified the virus in a way that made it more effective
选项是否正确？