An electron moving perpendicular to a magnetic field of 2.2 × 10-2 T moves in a circle of certain radius.

If the electron is moving with a speed of 1.5 × 107 m/s, what is the radius of the circle?
A)1.5 × 10-3 m
B)3.9 × 10-3 m
C)2.2 × 10-4 m
D)3.9 × 10-4 m
E)2.2 × 10-3 m

I would love an explanation!

The centripetal force exerted on electron :
Fs = m * v^2/R
This force is equal to the magnetic force :
Fm = e * B * v
Then e * v * B = m * v^2/r

SO R = m *v/ B * e

The radius of the circle :

R = m * v/ B * e ---m = mass v = speed B = magnetic field e = electron's charge

R = 9.11 x 10^-31 * 1.5 x 10^7/2.2 x 10^-2 1.6 x 10^-19
= 0.00388 m = 3.9 x 10^-3 m ( B)