Two parallel long wires $\mathrm{A}$ and B carry currents $\mathrm{I}_{1}$ and $\mathrm{I}_{2}$. $\left(\mathrm{I}_{2}<\mathrm{I}_{1}\right)$ when $\mathrm{I}_{1}$ and $\mathrm{I}_{2}$ are in the same direction the mag. field at a point mid way between the wires is $10 \mu \mathrm{T}$. If $\mathrm{I}_{2}$ is reversed, the field becomes $30 \mu \mathrm{T}$. The ratio $\left(\mathrm{I}_{1} / \mathrm{I}_{2}\right)$ is
(a) 1
(b) 2
(c) 3
(d) 4