Which of the following sets of ordered pairs is a function?
A
{(1, 5), (2, 6), (4, 5)}

B
{(5, 5), (6, 5), (5, 6)}

C
{(2, 3), (-4, 5), (2, 6)}

D
{(2, 3), (2, 6), (-2, 8)}