Light of wavelength $200\mathrm{nm}$ falls on a metal of work function $4.2\mathrm{eV}$. The fastest photoelectrons enter a uniform magnetic field of $1.0\mathrm{mT}$ perpendicular to their velocity. Use $h=6.626\times 10^{-34}\mathrm{J}\mathrm{s}$, $c=3.0\times 10^8\mathrm{m}{\mathrm{s}}^{-1}$, $m_e=9.1\times 10^{-31}\mathrm{kg}$, $e=1.6\times 10^{-19}\mathrm{C}$. The radius of their circular path is closest to

1. A3.38 mm
2. B4.78 mm
3. C9.57 mm
4. D0.00 m