Two thin long parallel wires separated by a distance $\mathrm{y}$ are carrying a current I Amp each. The magnitude of the force per unit length exerted by one wire on other is
(a) $\left[\left(\mu_{0} I^{2}\right) / y^{2}\right]$
(b) $\left[\left(\mu_{o} I^{2}\right) /(2 \pi \mathrm{y})\right]$
(c) $\left[\left(\mu_{0}\right) /(2 \pi)\right](1 / y)$
(d) $\left[\left(\mu_{0}\right) /(2 \pi)\right]\left(1 / \mathrm{y}^{2}\right)$