Someone claims the average high school GPA among incoming Berkeley freshmen is 3.9. A simple random sample of 150 freshmen is taken, however, and the average high school GPA in the sample is 3.5, with an SD of 0.5. Calculate the one-sample z test statistic based on this data, for the null hypothesis that the average in the population really is 3.9.

A) -0.8

B) -10

C) -2.5