$A$ copper rod of mass $m$ slides under gravity on two smooth parallel rails,with separation $l$ and set at an angle of $\theta$ with the horizontal. At the bottom,the rails are joined by a resistance $R$. There is a uniform magnetic field $B$ normal to the plane of the rails,as shown in the figure. The terminal speed of the copper rod is

• A
  $\frac{{mgR\cos \theta }}{{{B^2}{l^2}}}$
• B
  $\frac{{mgR\sin \theta }}{{{B^2}{l^2}}}$
• C
  $\frac{{mgR\tan \theta }}{{{B^2}{l^2}}}$
• D
  $\frac{{mgR\cot \theta }}{{{B^2}{l^2}}}$