A hypothesis will be used to test that a population mean equals 7 against the alternative that the population mean does not equal 7 with unknown variance. What are the critical values for the test statistic T₀ for the following significance levels and sample sizes?

(a) α = 0.01 and n = 20 

(b) α = 0.05 and n = 12

(c) α = 0.10 and n = 15