A horizontal wire is free to slide on the verticle rails of a conducting frame as shown in figure. The wire has a mass `m` and length `l` and the resistance of the circuit is `R`. If a uniform magnetic field `B` is directed perpendicular to the frame, the terminal speed of the wire as it falls under the force of gravity is
A. `(mgR)/(BI)`
B. `(mgi)/(BR)`
C. `(B^(2)I^(2))/(mgR)`
D. `(mgr)/(B^(2)I^(2)`
