Bill is testing a hypothesis about the mean (mu) of a population. He tests H0: mu = 35 vs H1: mu is not equal to 35. He commits a Type II error if

a. he rejects H0 when mu really is 35

b. he rejects H0 when mu really is 40

c. he does not reject H0 when mu really is 40

d. he does not reject H0 when mu really is 35

e. he rejects H0 when mu really is 30