4. which relation is a function?
a.
{(8, –1), (2, –1), (3, 8), (2, 5}
b.
{(1, 2), (1, 5), (1, –1), (1, 4)}
c.
{(–3, 4), (–2, 5), (0, 9), (0, 12)}
d.
{(7, 12), (1, –5), (3, –10), (2, –5)}