An electron moves in a region where the magnetic field is uniform and has a magnitude of 80 ?T. The electron follows a helical path which has a pitch of 9.0 mm and a radius of 2.0 mm. What is the speed of this electron as it moves in this region?
A. 48 km/s
B. 28 km/s
C. 20 km/s
D. 35 km/s
E. 8.0 km/s
Answer: D
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and a density of 
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