Determine the appropriate critical value(s) for each of the following tests concerning the population mean:
a. Upper-tailed test: a = 0.025; n = 25; σ = 3.0
b. Lower-tailed test: a = 0.05; n = 30; s = 9.0
c. Two-tailed test: a = 0.02; n = 51; s = 6.5
d. Two-tailed test: a = 0.10; n = 36; σ = 3.8