Three students take equivalent stress tests. Which is the highest z-score?

(a) A score of 254 on a test with a mean of 140 and a standard deviation of 60.
(b) A score of 260 on a test with a mean of 190 and a standard deviation of 40.
(c) A score of 233 on a test with a mean of 155 and a standard deviation of 30.