Which of these tables represents a function?

A. X: -1, -2, 0, 4 Y: 15, 6, 15, 6
B. X: 0, 6, 0, -2 Y: 4, 15, 6, -1
C. X: 4, -1, 6, 6 Y: 0, -2, -2, 15
D: X: 6, 4, 15, 4 Y: -2, 0, -1, 3