Two thin wire rings each having a radius R are placed at a distance d apart with their axes coinciding. The charges on the two rings are + q and - q. The potential difference between the centres of the two rings is :

• A

  $\frac{\mathrm{qR}}{4{\mathrm{\pi \epsilon }}_0{\mathrm{d}}^2}$

• B

  $\frac{\mathrm{q}}{2{\mathrm{\pi \epsilon }}_0}\left[\frac{1}{\mathrm{R}}-\frac{1}{\sqrt{{\mathrm{R}}^2+{\mathrm{d}}^2}}\right]$

• C

  Zero

• D

  $\frac{\mathrm{q}}{4{\mathrm{\pi \epsilon }}_0}\left[\frac{1}{\mathrm{R}}-\frac{1}{\sqrt{{\mathrm{R}}^2+{\mathrm{d}}^2}}\right]$