Ahmad's best test score for Chemistry has a z-score of 0.5 as compared to all students in his class. Which statement best interprets his z-score?

A)Ahmad's test score is 0.5 standard deviations above the mean for all students taking the same test.

B) Ahmad's test score is 0.5 points lower than all students taking the same test.

C) Ahmad's test score is 0.5 standard deviations below the mean for all students taking the same test.

D) Ahmad's test score is 0.5 standard deviations below the mean for all students taking the same test.