Which relation is a function?
A. {(1,2), (3,1),(4,2), (7, 2)}
B. {(1,2),(1,3),(4,5), (6, 1)}
C. {(0,1),(1,2), (0, 2), (2,3)}
D. {(1,4), (1,5), (2, 4), (3,6)}
E. {(2,4),(2,5), (2,6),(2,7)}