In Testing for the Differences Between the Means of 2 $n - 1$

In testing for the differences between the means of 2 independent populations where the variances in each population are unknown but assumed equal, the degrees of freedom are a) $n - 1$ .
b) $n _ { 1 } + n _ { 2 } - 1$ .
c) $n _ { 1 } + n _ { 2 } - 2$ .
d) $n - 2$ .