Please help with A, B and C. Full and good answer will be upvoted.

Problem 5 Cryptography a) Digital signature is used to provide the authentication to the owner of a message. There are basically four keys involved in a two-party communication: owner's public key, owner's private key, receiver's public key, and receiver's private key. Which key is used to sign a signature for a message? Which key is used to verify the signature received in together with a message? b) Another common method for integrity protection is the message authentication code (MAC). What are the major limitations of using a MAC (compared to the using of a digital signature)? c) In public key cryptography, a public key can be freely distributed. Please suggest a method on the distribution of the public key which allows a receiver to verify the public key's identity (i.e. to verify that the public key belongs to whom it claims to be from). Explain how a receiver would verify the public key