12. Which of the relations below is a function?
A. {(1, 1,), (2, 1), (3, 1), (4, 1), (5, 1)}
B. {(3, 1), (2, 2), (3, 3), (2, 4), (2,5)}
C. {(2, 1), (2, 2), (2, 3), (2, 4), (2,5)}
D. {(1, 1), (2, 2), (2, 3), (2, 4), (1,5)}