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

  • [JEE MAIN 2021]
  • A

    $\frac{b^{2}}{3 a^{3}}$

  • B

    $\frac{a+b}{3 a^{2}}$

  • C

    $\frac{a^{2}+b}{3 a^{3}}$

  • D

    $\frac{a+b^{2}}{3 a^{3}}$

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