Which set of ordered pairs (x, y) could represent a linear function?
A = {(-1,7), (1,5), (3,3), (6,1)}
B = {(0, 2), (2, 1), (4,3), (6,6)}
C = {(-1,6), (3,0), (5,-3), (9,-9)}
D = {(-1,2), (2, 1), (4,0), (8, -2)}