Which statement best describes spyware?

a$.$ It is software that secretly gathers information about users while they browse the Web.

b$.$ It is a programming routine built into a system by its designer to bypass system security and sneak back into the system later to access programs or files.

c$.$ It is an attack that floods a server with service requests to prevent legitimate users' access to the system.

d$.$ It is encryption security that manages transmission security on the Internet.