An electric current $\imath$ enters and leaves a uniform circular wire of radius $ \Re$ through diametrically opposite points. A charged particle q moving along the axis of the circular wire passes through the centre at speed $v$. The magnetic force acting on the particle when it passes through the centre is

Question

$\begin {array} {1 1} (a)\;QV\large\frac{\mu_0i}{2 \pi a} & \quad (b)\;QV\large\frac{\mu_0i}{4 a} \\ (c)\;QV\large\frac{\mu_0i}{2 a} & \quad (d)\;Zero \end {array}$