A wire cd of length l and mass m is sliding without friction on conducting rails ax and by as shown. The vertical rails are connected to each other with a resistance R between a and b. A uniform magnetic field B is applied perpendicular to the plane abcd such that cd moves with a constant velocity of

• A.             \[\frac{mgR}{Bl}\]
• B.             \[\frac{mgR}{{{B}^{2}}{{l}^{2}}}\]
• C.             \[\frac{mgR}{{{B}^{3}}{{l}^{3}}}\]
• D.             \[\frac{mgR}{{{B}^{2}}l}\]