Two thin rings of radius R are placed at a distance a between them such that their axes coincide. Potential difference between the centre of the ring will be , if charges on rings are $+q$ and $-q$

Question

$(A)\;\frac{2q}{\pi \in _0} \bigg[ \frac{1}{R} -\frac{1}{\sqrt {R^2+a^2}}\bigg]\\ (B)\;\frac{2q}{2\pi \in _0} \bigg[ \frac{1}{R} -\frac{1}{\sqrt {R^2+a^2}}\bigg] \\ (C)\; \frac{q}{2\pi \in _0} \bigg[ \frac{1}{R} -\frac{1}{\sqrt {R^2+a^2}}\bigg] \\ (D)\;none$