Two straight long conductors $A O B$ and $C O D$ are perpendicular to each other and carry currents $i_{1}$ and $i_{2}$. The magnitude of the magnetic induction at a point $P$ at a distance $a$ from the point $O$ in a direction perpendicular to the plane $A B C D$, is
(a) $\frac{\mu_{0}}{2 \pi a}\left(i_{1}+i_{2}\right)$
(b) $\frac{\mu_{0}}{2 \pi a}\left(i_{1}-i_{2}\right)$
(c) $\frac{\mu_{0}}{2 \pi a}\left(\hat{r}_{1}+\hat{z}_{2}\right)^{1 / 2}$
(d) $\frac{\mu_{0}}{2 \pi a} \frac{i_{1} i_{2}}{\left(i_{1}+i_{2}\right)}$