Determine the critical values that would be used in testing each of the following null hypotheses using the classical approach.
a. $\quad H_{o}: \rho=0$ vs. $H_{a}: \rho \neq 0,$ with $n=18$ and $\alpha=0.05$
b. $\quad H_{o}: \rho=0$ vs. $H_{a}: \rho>0,$ with $n=32$ and $\alpha=0.01$
c. $\quad H_{o}: \rho=0$ vs. $H_{a}: \rho<0,$ with $n=16$ and $\alpha=0.05$