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

Short Answer

In testing for differences between the means of 2 related populations where the variance of the differences is unknown, the degrees of freedom are a) $n - 1$ .
b) $n _ { 1 } + n _ { 2 } - 1$ .
c) $n _ { 1 } + n _ { 2 } - 2$ .
d) $n - 2$ .