Two concentric co-planar circular Loops of radii $\mathrm{r}_{1}$ and $\mathrm{r}_{2}$ carry currents of respectively $\mathrm{I}_{1}$ and $\mathrm{I}_{2}$ in opposite directions. The magnetic induction at the centre of the Loops is half that due to $\mathrm{I}_{1}$ alone at the centre. If $\mathrm{r}_{2}=2 \mathrm{r}_{1}$ the value of $\left(\mathrm{I}_{2} / \mathrm{I}_{1}\right)$ is
(a) 2
(b) $1 / 2$
(c) $1 / 4$
(d) 1