Which of the following reltions is not a function?

a
{(- 1, 4), (1, 6), (4, 10)}
b
{(- 1, 2), (1, 3), (- 1, 4)}
c
{(- 1, 6), (4, 7), (5, 6)}
d
{(- 1, 2), (1, 2), (3, 3)}