In each of the following instances, determine whether you would use a z- or t-statistic (or neither) to form a 90% confidence interval and then state the appropriate z- or t-statistic value for the confidence interval.

a. Random sample of size n = 32 from a normal distribution with a population mean of 60 and population standard deviation of 4.

b. Random sample of size n = 108 from an unknown population.

c. Random sample of size n = 12 from a normal distribution with sample mean of 83 and sample standard deviation of 2.

d. Random sample of size n = 24 from a normal distribution with unknown mean and sample standard deviation of 3.