Computer virus: A program, an executable piece of code, is a program that can spread to other programs through disks, tapes, and networks as a medium. It can "infect" other programs. Just like biological viruses, computer viruses have a unique ability to replicate. A latent, contagious and destructive program that achieves its own replication and exists with the aid of a certain carrier. Based on the above definition, which of the following is a computer virus? ['Antivirus software.', 'Calculator program code that can provide common mathematical operations in Windows.', 'Infectious diseases.', "A piece of program code that changes the contents of the computer's registry."]