The grades of a science test are normally distributed. thanh finds that his grade on the test has a z-score of –2.5. which of the following statements must be true?

a. thanh’s grade is 2.5 standard deviations below the mean of the test grades.

b. thanh’s grade is 2.5 points below the mean of the test grades.

c. thanh’s grade is 2.5 standard deviations above the mean of the test grades.

d. thanh’s grade is 2.5 points above the mean of the test grades.