Which relation defined by a set of ordered pairs is not a function?   {(1, –1), (4, –2), (9, –3), (16, –4), (25, –5)} {(1, 1), (2, 4), (3, 9), (4, 16), (5, 25)} {(1, –1), (2, –2), (0, 0), (1, 1), (2, 2)} {(–1, 1), (–2, 4), (–3, 9), (–4, 16), (–5, 25)}

The answer is:  [C]:  

"{(1, –1), (2, –2), (0, 0), (1, 1), (2, 2)} " {(1, –1), (2, –2), (0, 0), (1, 1), (2, 2)}" .

This set of ordered pairs is NOT a function because their is one x-coordinate in this set that has multiple y-coordinates;  
                               specifically:  "(1, -1)" ; and "(1, 1)".

       All of the other answer choices given are functions.