Two identical long conducting wires $AOB$ and $COD$ are placed at right angle to each other, with one above other such that '$O$ ' is their common point. The wires carry $I_1$ and $I_2$ currents, respectively. Point '$P$ ' is lying at distance '$d$ ' from '$O$ ' along a direction perpendicular to the plane containing the wires. The magnetic field at point '$P$ ' will be:

1. A
   $\frac{{\mu }_0}{2\pi d}(I_1^2+I_2^2{)}^{1/2}$
2. B
   $\frac{{\mu }_0}{2\pi d}(I_1+I_2)$
3. C
   $\frac{{\mu }_0}{2\pi d}(I_1^2-I_2^2)$
4. D
   $\frac{{\mu }_0}{2\pi d}(I_1^2+I_2^2{)}^{1/2}$