Which set of ordered pairs represents a function
A) {(2, 2), (1, 5), (2, 2), (1, 3), (8, 1)}
B) {(3, 1), (7, 1), (6, 1), (9, 1), (2, 1)}
C) {(6, 8), (5, 2), (2, 5), (1, 3), (2, 9)}
D) {(3, 1), (6, 3), (3, 2), (3, 3), (1, 1)}