$(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 $