Which set of ordered pairs represents y as a function of x?
O {(3, 3), (3, 4), (4,3), (4,4)}
O {(2,-1), (4,-2), (6,-3), (8,4)}
O {(0,0), (1, 1), (1,0), (2, 1)}
O {(1,-5),(1,5), (2, -10), (2, -15)}