Which of the following does not represent a function?

a
{(3, -7), (2, -6), (5, -5), (7, -4)}

b
{(-3, -1), (-1, 2), (4, 2), (5, -3)}

c
{(4, 2), (5, 4), (6, 8), (7, -2)}

d
{(-1, 2), (-4, 5), (-5, -4), (-1, 8)}