If b is very small as compared to the value of a, so that the cube and other higher powers of `(b)/(a)` can be neglected in the identity `(1)/(a-b)+(1)/(a-2b)+(1)/(a-3b)+...+(1)/(a-nb)=alphan+betan^(2)+lambdan^(3)`, then the value of `lambda` is :

Options

A. `(a^(2)+b)/(3a^(3))`
B. `(a+b)/(3a^(2))`
C. `(b^(2))/(3a^(3))`
D. `(a+b^(2))/(3a^(3))`