An electron moves in a uniform circular motion under the action
of an external magnetic field.
Take the charge of the electron to be q = -1.602 x 10-^19 C, the
speed of the electron to be y = 2.8 x 10^7 m/s, and the magnitude
of the external magnetic field to be B = 2.1 x 10^-3 T.
Calculate the radius R of the circle formed by the electron in its
trajectory during its displacement and choose the correct
option.