Read the passage carefully and choose the best answer to each question out of the four alternatives.

A computer virus is a type of malware that, when executed, replicates by inserting copies of itself into other computer programs, data files, or the boot sector of the hard drive. However, not all viruses carry a destructive payload or attempt to hide themselves—the defining characteristic of viruses is that they are self-replicating computer programs which install themselves without the user’s consent. Virus writers use social engineering and exploit detailed knowledge of security vulnerabilities to gain access to their host’s computing resources. The vast majority of viruses (over 99%) target systems running Microsoft Windows employing a variety of mechanisms to infect new hosts, and often using complex anti-detection/stealth strategies to evade antivirus software. Motives for creating viruses can include seeking profit, desire to send a political message, personal amusement, to demonstrate that a vulnerability exists in software, for sabotage and denial of service, or simply because they wish to explore artificial life and evolutionary algorithms.