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

Multiple Choice

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) $n - 2.$