Four long insulated current carrying wires, cross each other as shown in the figure. If the separation between any pair of parallel wires is d, the magnetic induction at O is

• A

  $\frac{3{\mathrm{\mu }}_{\mathrm{o}}\mathrm{I}}{\mathrm{\pi d}},$ out ward

• B

  $\frac{4{\mathrm{\mu }}_{\mathrm{o}}\mathrm{I}}{\mathrm{\pi d}},$ out ward

• C

  $\frac{3{\mathrm{\mu }}_{\mathrm{o}}\mathrm{I}}{\mathrm{\pi d}},$ inward

• D

  $\frac{6{\mathrm{\mu }}_{\mathrm{o}}\mathrm{I}}{\mathrm{\pi d}},$ out ward