Determine which set of ordered pairs is a function.

a.{(-1,3),(1,4),(-1,5)}
b. {(5,3),(-6,3),(-2,4)}
c. {(0.5, 1.6),(1, 1.75),(1.5, 2.3), (0.5, 3.6)}
d. {(1,2), (-5,4),(3,6),(-5,2)}