JEE Math Practice Question

If b is very small as compared to the value of a, so that the cube and other higher powers of $${b \over a}$$ can be neglected in the identity $${1 \over {a - b}} + {1 \over {a - 2b}} + {1 \over {a - 3b}} + ..... + {1 \over {a - nb}} = \alpha n + \beta {n^2} + \gamma {n^3}$$, then the value of $$\gamma$$ is :

1. A.$${{{a^2} + b} \over {3{a^3}}}$$
2. B.$${{a + b} \over {3{a^2}}}$$
3. C.$${{{b^2}} \over {3{a^3}}}$$
4. D.$${{a + {b^2}} \over {3{a^3}}}$$