Two thin wires 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

$$QR/4\pi \varepsilon_0 d^2$$

B

$$\frac{Q}{2\pi \varepsilon_0}\left[\frac{1}{R} - \frac{1}{\sqrt{R^2 + d^2}}\right]$$

C

zero

D

$$\frac{Q}{4\pi \varepsilon_0}\left[\frac{1}{R} - \frac{1}{\sqrt{R^2 + d^2}}\right]$$