In the Arrangement Shown, a Conducting Bar of Negligible Resistance

In the arrangement shown, a conducting bar of negligible resistance slides along horizontal, parallel, frictionless conducting rails connected as shown to a 2.0-Ω resistor. A uniform 1.5-T magnetic field is perpendicular to the plane of the paper. If L = 60 cm, at what rate is thermal energy being generated in the resistor at the instant the speed of the bar is equal to 4.2 m/s?

A) 8.6 W
B) 7.8 W
C) 7.1 W
D) 9.3 W
E) 1.8 W