Solve this:
Q.56. $\sum _{i=1}^{\infty } \sum _{j=1}^{\infty } \sum _{k=1}^{\infty }\frac{1}{a^{i+j+k}}$ is equal to, where $\left|a\right|>1$
(A) ${\left(a-1\right)}^{-3}$
(B) $\frac{3}{a-1}$
(C) $\frac{1}{a^3-1}$
(D) None of these