If the expansion in powers of $x$ of the function $\frac{1}{(1-a x)(1-b x)}$ is $a_{0}+a_{1} x+a_{2} x^{2}+a_{3} x^{3}+\ldots$, then $\mathrm{a}_{\mathrm{n}}$ is
(A) $\frac{b^{n}-a^{n}}{b-a}$
(B) $\frac{a^{n}-b^{n}}{b-a}$
(C) $\frac{a^{n+1}-b^{n+1}}{b-a}$
(D) $\frac{b^{n+1}-a^{n+1}}{b-a}$