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Two bar magnets with magnetic moments 2 M 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 earth’s magnetic field with magnet having magnetic moment 2M making and angle q with the magnetic meridian such that

(a) $\theta ={\mathrm{tan}}^{-1}\left(\frac{1}{\sqrt{3}}\right)$            (b) $\theta ={\mathrm{tan}}^{-1}\left(\sqrt{3}\right)$

(c) $\theta ={\mathrm{tan}}^{-1}\left(\frac{1}{2}\right)$               (d) $\theta ={\mathrm{tan}}^{-1}\left(\frac{3}{4}\right)$