A copper rod of mass m slides under gravity on two smooth parallel rails, with separation l and set at an angle of $$\theta$$ with the horizontal. At the bottom, rails are joined by a resistance R. There is a uniform magnetic field B normal to the plane of the rails, as shown in the figure. The terminal speed of the copper rod is:

A

$$\frac{mgR\cos\theta}{B^2 l^2}$$

B

$$\frac{mgR\sin\theta}{B^2 l^2}$$

C

$$\frac{mgR\tan\theta}{B^2 l^2}$$

D

$$\frac{mgR\cot\theta}{B^2 l^2}$$