please answer correctly

a. Rank from best to worst: (10 Points) (i) Encrypt-and-authenticate (ii) Encrypt-then-authenticate (iii) Authenticate-then-Encrypt Explain in a sentence or two why each of these schemes is good/bad b. Consider a group of 50 people in a room who wish to establish pairwise secure communications in the future. How many keys need to be exchanged in total: (i) Using symmetric cryptography? ( 3 points) (ii) Using public-key cryptography? (2 points)