The distance between two thin long straight parallel conducting wires is $b$. On passing the same current $i$ in them, the force per unit length between them will be
(A) Zero
(B) $\frac{\mu_{0} i^{2}}{2 \pi b}$
(C) $\frac{\mu_{0} i}{2 \pi b}$
(D) $\frac{\mu_{0} i}{2 \pi}$