Which of the following relations is a function?

A.
(1, 4), (-3, 6), (1, 3), (-7, 2)

B.
(1, 0), (-3, 3), (7, 1), (-3, 5)

C.
(1, 4), (-3, 2), (7, 1), (-7, 2)

D.
(7, 1), (-3, 4), (1, 1), (7, 2)
this is study island understanding functions