A charge $+Q$ is uniformly distributed over a thin ring of the radius $R$. The velocity of an electron at the moment when it passes through the centre $O$ of the ring, if the electron was initially at rest at a point $A$ which is very far always from the centre and on the axis of the ring is :
(a) $\sqrt{\left(\frac{2 K Q e}{m R}\right)}$
(b) $\sqrt{\left(\frac{K Q e}{m}\right)}$
(c) $\sqrt{\left(\frac{K m e}{Q R}\right)}$
(d) $\sqrt{\left(\frac{K Q e}{m R}\right)}$