A battery is connected between two points $A$ and $B$ on the circumference of a uniform conducting ring of radius $r$ and resistance $R$. One of the arcs $AB$ of the ring subtends an angle $\theta$ at the centre. The intensity of the magnetic field at the centre due to a constant current in the ring is

Question

$\begin {array} {1 1} (a)\;Proportional\: to (180^{\circ}- \theta) & \quad (b)\;Inversely\: proportional\: to\: r \\ (c)\;Zero\: only\: if\: \theta = 180^{\circ} & \quad (d)\;Zero\: for \: all\: values\: of\: \theta \end {array}$