Aproton with speed v moves in a circle of radius rp perpendicular to a uniform magnetic field of magnitude b. an α-particle moves with the same speed v in a circle of radius ra perpendicular to the same field. given that the alpha particle has four times the mass of the proton and twice its charge, what is the relation between ra and rp?
a. ra = rp/4
b. ra = (1/2) rp/2
c. ra = rp
d. ra = 2 rp
e. ra = 4 rp