In an experiment, electrons are accelerated, from rest, by applying a voltage of 500 V. Calculate the radius of the path if a magnetic field 100 mT is then applied. [Charge of the electron $= 1.6 \times 10 ^ { - 19 } \mathrm { C }$, Mass of the electron $= 9.1 \times 10 ^ { - 31 } \mathrm {~kg}$]\\
(1) $7.5 \times 10 ^ { - 3 } \mathrm {~m}$\\
(2) $7.5 \times 10 ^ { - 2 } \mathrm {~m}$\\
(3) 7.5 m\\
(4) $7.5 \times 10 ^ { - 4 } \mathrm {~m}$