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