Which relation is a function?


{(−2, 1), (−2, 2), (−2,  7), (−2, 8)}
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{(−2, 2), (−7, 7), (−1,  1), (−8, 8)}

{(1, −7), (2, −8), (−2,  7), (1, 8)}

{(−2, 1), (−7, −1), (2,  8), (−7, −8)}