Which set of ordered pairs represents y as a function of ?
A {(2,5), (3, 1), (2, 1), (4,7)
B {(3, 2), (4,3), (5,2), (2,6)}
C {(1, 3), (3,5), (2,5), (1,6)}
D {(4,7), (4, 6), (4,4). (4,1)}