PLEASE HELP
Which relation is a function?
A {(8, 1), (4, 1), (0, 1), (-15, 1);
B {(-2, -12), (-2,0), (-2,4), (-2, 11)}
C (-4,-6), (-3, -2), (1, -2), (1, 0);
D ((0, 1), (0, 2), (1, 2), (1,3)}