contestada

Determine which relation is a function.
A. {(1,2), (2,3), (3,4), (4,5)}
B. {(2,2), (3,2), (4,2), (1,2)}
C. {(5,7), (5,8), (5,1), (5,3)}
D. {(1,2), (5,8), (1,4), (3,8)}