An Electron Is Launched with Velocity in a Uniform Magnetic $\theta$

Multiple Choice

An electron is launched with velocity in a uniform magnetic field .The angle $\theta$ between and is between 0 and 90^o.As a result, the electron follows a helix, its velocity vector returning to its initial value in a time interval of:

A) 2πm/eB
B) 2πmv/eB
C) 2πmv sin $\theta$ /eB
D) 2πmv cos $\theta$ /eB
E) none of these