A proton moving with a velocity 3 x 10 m/s enters a magnetic field of 0.3 tesla at an angle of 30 with the field. The radius of curvature of its path will be (e/m for proton = 10 C/kg) (a) 2 cm (c) 0.02 cm (b) 0.5 cm (d) 1.25 cm