Two thin long parallel wires separated by a distance b carrying a current i ampere each. The magnitude of the force per unit length exerted by one wire on the other is

• A

  $\frac{{\mathrm{\mu }}_0{\mathrm{i}}^2}{{\mathrm{b}}^2}$

• B

  $\frac{{\mathrm{\mu }}_0{\mathrm{i}}^2}{2\mathrm{\pi b}}$

• C

  $\frac{{\mathrm{\mu }}_0\mathrm{i}}{2\mathrm{\pi b}}$

• D

  $\frac{{\mathrm{\mu }}_0\mathrm{i}}{2{\mathrm{\pi b}}^2}$