Which of the following relations represents a function?
a. { (2, 3), (1, 3), (3, 3) } c. { (1, 3), (2, 3), (1, 4) }
b. { (1, 3), (2, 3), (2, 4) } d. { (2, 2), (2, 3), (2, 1) }