$A$ wire $AB$ is carrying a steady current $I_1$ and is kept on a table. Another wire $CD$ carrying current $I_2$ is held directly above it at a distance $r$. When the wire $CD$ is left free and it remains suspended at its position, find its mass per unit length. (Given: $g$ = acceleration due to gravity, $\mu_0$ = permeability of free space)

• A
  $\frac{\mu_0 I_1 I_2}{2\pi r g}$
• B
  $\frac{\mu_0 I_1 I_2}{4\pi r g}$
• C
  $\frac{\mu_0 I_1 I_2}{\pi r g}$
• D
  $\frac{\mu_0 I_1 I_2}{\pi r^2 g}$