Which data set contains an outlier?

{6.1, 6.8, 7.3, 8, 9.1, 9.5}

{14, 15, 16, 20, 22, 25, 51}

{30, 31, 31, 32, 34, 37}

{11, 12, 12, 12, 13, 13, 14, 14}