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