[ PLEASE HELP ASAP, I DONT GET IT ] Which is a function?
a.) {(1000, 10), (1000, 12), (1000, 16), (100, 5), (100, 7), (78, 3), (90, 5)}

b.) {(12, 3), (11, 2), (10, 1), (9, 0), (8, 1), (7, 2), (6, 3)}

c.) {(6, 3), (5, 2), (4, 1), (3, 0), (4, –1), (5 ,–2), (6 ,–3)}

d.) {(7, 2), (100, 10), (13, –7), (7, 9), (10, 100), (4, –2), (5, 5)}