(a) When Alice sends a message to Bob, what key will she use to encrypt the message?...

Question:

(a) When Alice sends a message to Bob, what key will she use to encrypt the message?
(b) Why is "the public key" not a good answer to Question 21a?
(c) What key will Bob use to decrypt the message?
(d) Why is "the private key" not a good answer to Question 21b?
(e) In a classroom with 30 students and a teacher, how many public keys will there be?
(f) How many private keys?