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)}