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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

(1) $\frac{mgR}{Bl}$

(2) $\frac{mgR}{{B}^{2}{l}^{2}}$

(3) $\frac{mgR}{{B}^{3}{l}^{3}}$

(4) $\frac{mgR}{{B}^{2}l}$