The daily output of a certain department within

The daily output of a certain department within an industrial plant is presumed to be normally distributed and has an average of 85 units. Twenty-five days are selected at random, and the output for each day is recorded. The average output calculated from this sample is 81 units, and the standard deviation is 9 units. What is the critical value of the test statistic at the 1% level of significance if the alternate hypothesis states the mean is not equal to 85?

a.            z = ± 2.323

b.            z = ± 2.575

c.             t = ± 2.787

d.            t = ± 2.797