2. Which of the following are functions on the domain A = {1, 2, 3}? Explain why or why not. (a) f = {(1, 1), (2, 2), (3,3) } (b) f = {(1, 1), (2, 1), (3, 1) } (c) f = {(1,3), (3, 2) } (d) f = {(1, 1), (1, 2), (2, 1), (3, 2) }