Which sets of ordered pairs represent relations that are functions? you may choose more than one correct answer.

a. {(1, 0), (2, 0), (3, 0), (4, 0)}
b. {(2, 3), (2, 4), (2, 5), (2, 6)}
c. {(1, 1), (2, 2), (3, 3), (4, 4)}
d. {(1, 3), (-1, 4), (1, 4), (-1, 3)}
e. {(6, -6), (-6, 6), (5, -5), (-5, 5)}