In the context of the common intentional security threats, which statement best describes a worm?

a$.$ It attaches itself to a host program to spread to other files in a computer.

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 data.

c$.$ It travels from computer to computer in a network, but it does not usually erase data.

d$.$ It floods a network or server with service requests to prevent legitimate users' access to the system.