Complete parts (a) through (c) below. (a) Determine the critical value(s) for a right-tailed test of a population mean at the a=.01 level of significance with 15 degrees of freedom. (b) Determine the critical value(s) for a left-tailed test of a population mean at the a=.05 level of significance based on a sample size of n=20 (c) Determine the critical value(s) for a two-tailed test of a population mean at the a=.10 level of significance based on a sample size of n=16