Determine which relation is a function.

a.
{(–3, 2), (–2, 3), (–1, 2), (0, 4), (1, 1)}

b.
{(–3, 2), (–1, 3), (–1, 2), (0, 4), (1, 1)}

c.
{(–3, 2), (–2, 3), (–1, 1), (0, 4), (0, 1)}

d.
{(–3, 3), (–2, 3), (–1, 1), (0, 4), (0, 1)}