# If the expansion in power of $x$ of the function $\frac{1}{ (1-ax) (1-bx)}$ <br> $a_{_0} + a_1x + a_2x^2 + a_3x^3 + ....,$ then $a_n$ is :
( A ) $\frac{b^{n+1} - a^{n+1} }{b-a}$
( B ) $\frac{a^n - b^n}{b-a}$
( C ) $\frac{b^n - a^n}{b-a}$
( D ) $\frac{a^{n+1} - b^{n+1} }{b-a}$