Which of the following statements is true of a worm?
a. It is an independent program that can spread itself without attaching itself to a host program.
b. It is usually hidden inside a popular program, but it is not capable of replicating itself.
c. It enables a system designer to bypass the security of a system and sneak back into the system later to access files.
d. It floods a network or server with service requests to prevent legitimate users’ access to the system.