Charges +q and -q are placed on vertices B and C of isosceles triangle. The potential at vertex A is :

Question

 

$(A)\;\frac{1}{4 \pi \in_0} \frac{2q}{\sqrt {a^2+b^2}} \\ (B)\;zero \\ (C)\; \frac{1}{4 \pi \in_0} \frac{q}{\sqrt {a^2+b^2}} \\ (D)\;None$