Which relation is a function? NEED HELP ASAP PLS

{(0, 2), (0, 4), (0, 6), (0, 8)}

{(1, 2), (2, 3), (3, 4), (4, 5)}

{(1, 2), (2, 1), (1, 3), (3, 1)}

{(1, 2), (3, 4), (5, 6), (1, 8)}

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