An Electron Moves in a Region Where the Magnetic Field

Multiple Choice

An electron moves in a region where the magnetic field is uniform, has a magnitude of 60 μT, and points in the positive x direction. At t = 0 the electron has a velocity that has an x component of 30 km/s, a y component of 40 km/s, and a z component of zero. What is the radius of the resulting helical path?

A) 4.7 mm
B) 18 mm
C) 3.8 mm
D) 2.8 mm
E) 5.7 mm