Two thin long parallel wires, $W_1$ and $W_2$ separated by distance '$a$',carry currents $i$ and $3i$ respectively in the same direction. The magnitude of the force per unit length exerted by wire $W_1$ on wire $W_2$ is

• A
  $\frac{\mu_0 i^2}{2\pi a}$
• B
  $\frac{\mu_0 i^2}{\pi a}$
• C
  $\frac{3\mu_0 i^2}{2\pi a}$
• D
  $\frac{2\mu_0 i^2}{3\pi a}$