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 :

Question

( A ) $\frac{q}{2 \pi \varepsilon_0} [\frac{1}{R} - \frac{1}{\sqrt{R^2+d^2}}]$
( B ) zero
( C ) $\frac{qR}{4 \pi \varepsilon_0 d^2}$
( D ) $\frac{q}{4 \pi \varepsilon_0} [\frac{1}{R} - \frac{1}{\sqrt{R^2+d^2}}]$