Select the correct answer.
Which of the following sets of ordered pairs represents a function?
A.
{(-12,8), (-15,8), (-5,-8), (-12,-8)}
B.
{(8,-9), (-8,-5), (8,-7), (-8,-6)}
C.
{(13,-3), (13,0), (13,-1), (13,-1)}
D.
{(-9,8), (-5,-8), (-7,8), (-6,-8)}