A conducting rod of mass $m$ and length $L$ is moving down a smooth incline plane on inclination $\theta$ with constant velocity $v$. A current $i$ is flowing in the conductor in a direction perpendicular to the paper inwards. A vertically upward magnetic field $\overrightarrow B$ exists in space. Its magnitude is

Question

$\begin {array} {1 1} (a)\;\large\frac{mg}{il}\sin \theta & \quad (b)\;\large\frac{mg}{il}\cos \theta \\ (c)\;\large\frac{mg}{il}\tan \theta & \quad (d)\;\large\frac{mg}{il} \end {array}$