contestada

Which relation is NOT a function?
A. {(1, 4), (2, 2), (3,5), (4, 3), (5,1)}
B. {(3,5), (2,5), (1,5), (0,5), (-1,5)}
C. {(3, -1), (5,0), (1, 2), (4,4), (2, 2)}
D. {(1,4), (2,5), (3,6), (2, 2), (1, 1)}