Which of the following relations is a function? A. (7, 1), (-2, 4), (3, 1), (7, 2) B. (3, 4), (-2, 6), (3, 3), (-7, 2) C. (3, 4), (-2, 2), (7, 1), (-7, 2) D. (3, 0), (-2, 3), (7, 1), (-2, 5)