What is a key difference between a MAC and a digital signature?

Select one:

a$.$ A MAC requires a public key, while a digital signature uses a private key

A MAC can be verified by anyone, while a digital signature requires a shared

b$.$ secret

A MAC is generated using a shared secret key, while a digital signature uses a

c$.$ private$/$public key pair

d$.$ A MAC provides encryption, while a digital signature provides decryption