A conducting rod of length $\ell$ [cross-section is shown] and mass $\mathrm{m}$ is moving down on a smooth inclined plane of inclination $\theta$ with constant speed v. A vertically upward mag. field $\mathrm{B}^{-}$ exists in upward direction. The magnitude of mag. field $B^{-}$ is(a) $[(\mathrm{mg} \sin \theta) /(\mathrm{I} \ell)]$
(b) $[(\mathrm{mg} \cos \theta) /(\mathrm{I} \ell)]$
(c) $[(\mathrm{mg} \tan \theta) /(\mathrm{I} \ell)]$
(d) $[(\mathrm{mg}) /(\mathrm{I} \ell \sin \theta)]$