Alpha Particles,each Having a Charge of +2e and a Mass

Multiple Choice

Alpha particles,each having a charge of +2e and a mass of 6.64 × 10^-27 kg,are accelerated in a uniform 0.80-T magnetic field to a final orbit radius of 0.30 m.The field is perpendicular to the velocity of the particles.How long does it take an alpha particle to make one complete circle in the final orbit? (e = 1.60 × 10^-19C)

A) 0.15 μs
B) 0.25 μs
C) 0.33 μs
D) 0.40 μs