Question

Which of the following relations is a function?
A) {(3,7), (5,8), (2,1), (1,3),(5,9)}
B) {(-1,2), (-2,-4), (-4,-5), (-3,-4), (-2,-2)}
C) {(2,1), (1,1), (-3,3), (-4,3), (0,0)}
D) {(7,1), (7,2), (7,3), (7,4), (7,5)}