Which relation is not a function?

• A. {(2,5), (3,6),(4,7), (5,8)}
• B. {(6,-2), (-4,6), (-2,4), (1,0)}
• C. {(-1, 5), (-2,5), (-3,5), (-4,5)}
• D. {(0,-2), (1,0), (-1,-3), (0,-1)}