An arrangement of three parallel straight wires placed perpendicular to plane of paper carrying same current ‘$I$ ’ along the same direction is shown in Fig. Magnitude of force per unit length on the middle wire ‘$B$ ’ is given by:

1. A
   $\frac{{\mu }_0{I}^2}{2\pi d}$
2. B
   $\frac{{\mu }_0{I}^2}{\sqrt{2}\pi d}$
3. C
   $\frac{\sqrt{2}{\mu }_0{I}^2}{\pi d}$
4. D
   $\frac{{\mu }_0{I}^2}{\sqrt{2}\pi d}$