Two bar magnets with magnetic moments $2M$ and $M$ are fastened together at right angles to each other at their centres to form a crossed system,which can rotate freely about a vertical axis through the centre. The crossed system sets in the Earth's magnetic field with the magnet having magnetic moment $2M$ making an angle $\theta$ with the magnetic meridian such that:

• A
  $\theta = \tan^{-1}\left(\frac{1}{\sqrt{3}}\right)$
• B
  $\theta = \tan^{-1}(\sqrt{3})$
• C
  $\theta = \tan^{-1}\left(\frac{1}{2}\right)$
• D
  $\theta = \tan^{-1}\left(\frac{3}{4}\right)$