Question

Which relation is a function
A. {(1, 2); (1, 3); (1, 4); (1, 5)}
B. {(1, 2); (2, 2); (3, 2); (4, 2)}
C. {(1, 2); (2, 3); (3, 4); (2, 5)}
D. {(1, 2); (2, 3); (3, 4); (1, 5)}