8. Given that the sum of the reciprocals of the roots of the equation $40 x^{2}+39 x-1=0$ is the $m$-th term of the arithmetic sequence $1,3,5, \cdots$, and the product of the reciprocals of the roots is the sum of the first $n$ terms of the geometric sequence $2,-6,18, \cdots$, then the value of $m+n$ is ( ).
A. 20
B. 22
C. 24
D. 26
8. C. Let $x_{1}, x_{2}$ be the roots of the equation $40 x^{2}+39 x-1=0$, then $x_{1}+x_{2}=-\frac{39}{40}, x_{1} x_{2}=-\frac{1}{40}, \therefore \frac{1}{x_{1}}+\frac{1}{x_{2}}=39$, $\frac{1}{x_{1} x_{2}}=-40$, from $39=1+2(m-1)$ we get $m=20$, from $-40=\frac{2\left[1-(-3)^{n}\right]}{1-(-3)}$, we get $n=4, \therefore \quad m+n=24$, hence the answer is C.