An oil drop of mass $m$, containing $n$ extra electrons, is kept at rest in the uniform electric field between two horizontal plates spaced a distance $d$ apart. The voltage across the plates is $V$, the elementary charge is $e$ and the acceleration of free fall is $g$. Determine the value of $n$.
- A$\frac{eV}{mgd}$
- B$\frac{mgd}{eV}$
- C$\frac{meV}{gd}$
- D$\frac{gd}{meV}$