State the total degrees of freedom for the following $t$ tests:
a. $n=12$ for a one-sample $t$ test
b. $d f_{1}=12, n_{2}=19$ for a two-independent-sample $t$ test
c. Critical value $=63.657$ for a two-tailed test, $\mathrm{a}=.01$
d. Critical value $=1.645$ for a one-tailed test, $\mathrm{a}=.05$