A horizontal wire AB of length l is free to slide on a vertical conducting frame as shown. The wire has mass m and length l and resistance of circuit is R. The system is placed in a uniform magnetic field B directed perpendicular to the frame. The wire after falling a distance under the influence of gravity aquires a terminal speed equal to

Question

$(a)\;\large\frac{B^2l^2}{mgR} \\ (b)\;\large\frac{mgR}{b^2l^2} \\(c)\;\large\frac{mgR^2}{B^2l} \\(d)\;\large\frac{mgl}{BR} $