$A$ coil in the shape of an equilateral triangle of side $l$ is suspended between the pole pieces of a permanent magnet such that $\vec{B}$ is in the plane of the coil. If due to a current $i$ in the triangle a torque $\tau$ acts on it,the side $l$ of the triangle is

• A
  $\frac{2}{\sqrt{3}}\left(\frac{\tau}{B i}\right)$
• B
  $2\left(\frac{\tau}{\sqrt{3} B i}\right)^{1 / 2}$
• C
  $\frac{2}{\sqrt{3}}\left(\frac{\tau}{B i}\right)^{1 / 2}$
• D
  $\frac{1}{\sqrt{3}} \frac{\tau}{B i}$