(20 points) determine which set of ordered pairs (x, y) represents y as a function of x.
a.) {(4, -2), (-8, -1), (-8, 4), (-1, -8)}
b.) {(4, -2), (-8, 4), (-1, -2), (-8, -4)}
c.) {(4, -2), (-2, -8), (4, -1)}
d.) {(4, -2), (-2, 4), (-1, -1)}