As shown in Fig. there are two semicircles of radii $\mathrm{r}_{1}=12 \mathrm{~cm}$ and $r_{2}=10 \mathrm{~cm}$ in which 4 Amp current is following The mag. field at the centre ${ }^{n} \mathrm{O}^{n}$ is(a) $(55 / 3) \mu_{0}$ going inside
(b) $(3 / 55) \mu_{0}$ going outside
(c) $(6 / 55) \mu_{0}$ going inside
(d) $(12 / 55) \mu_{0}$ going inside