An arrangement of three parallel straight wires placed perpendicular to plane of papei carrying same current'I' along the same direction is shown in the figure . Magnitude of force per unit length on the middle wire'B' is given by

Question

$\begin{array}{1 1} (1) \frac{\mu_0 i^2}{2 \pi d} \\ (2) \frac{2 \mu_0 i^2}{ \pi d} \\ (3) \frac{\sqrt 2 \mu_0 i^2}{ \pi d}\\ (4) \frac{\mu_0 i^2}{\sqrt 2 \pi d}\end{array} $