qWhich set of ordered pairs represents a function?

a.{(4, 1), (4, 2), (4, 4), (5, 4), (6, 4), (7, 4), (8, 1), (8,2)}

b.{(−13, 4), (7, −15), (−13, 9), (6, −12), (−18, 0), (−1, 5)}

c.{(−4, 3), (5, −9), (11, 4), (9, 6), (5, −3), (8, − 9), (5, 4)}

d.{(1, 4), (2, 4), (3, 4 ), (4, 4), (5, 4), (6, 4), (7,4), (8,4)}​