Which relation represents a function?

A: {(5, -9), (3, -11), (0, -13), (5, -15)}

B: {(-4, -13), (-1, -7), (2, -1), (2,5), (6, 7)}

C:{(-8.1, 2), (-7.6, 2), (-7.1, 2), (-6.6, 2)}

D: {(-1, ), (-1, 1), (1, 2), (1, 4), (3, 8)}​