Which of the following is the MAIN difference between a rootkit and a virus?

Neither rootkits nor viruses are primarily defined by encrypting the OS or documents - encryption is a characteristic of ransomware, not a defining difference between rootkits and viruses.

While rootkits often seek elevated privileges, viruses are not limited to executing as regular users - many viruses can escalate privileges or execute with administrative rights depending on context.

BIOS modification is a characteristic of some advanced bootkits (a subtype of rootkit) but is not a defining or common characteristic of rootkits as a whole, and viruses can theoretically access the BIOS under certain conditions.

Rootkits are designed to gain privileged (kernel-level) access to the operating system, allowing them to hide processes, files, and registry entries from the OS and security tools. Viruses, by contrast, attach themselves to executable files or documents and replicate at the user/file level without necessarily requiring elevated system privileges.