Calculate the Degrees of Freedom Associated with a Small-Sample Test $\left( \mu _ { 1 } - \mu _ { 2 } \right)$

Calculate the degrees of freedom associated with a small-sample test of hypothesis for $\left( \mu _ { 1 } - \mu _ { 2 } \right)$ , assuming $\sigma _ { 1 } ^ { 2 } \neq \sigma _ { 2 } ^ { 2 }$ and $n _ { 1 } = 13 , n _ { 2 } = 12 , s _ { 1 } = 1.3 , s _ { 2 } = 1.5$ .

A) 23
B) 25
C) 12
D) 11