In the Bohr model of a hydrogen atom, the centripetal force is furnished by the coulomb attraction between the proton and the electron. If ${\mathrm{a}}_{0}$ is the radius of the ground state orbit, m is the mass, e is the charge on the electron and ${\mathrm{\epsilon }}_{0}$ is the vacuum permittivity, the speed of the electron is
(a) 0                         (b) $\frac{\mathrm{e}}{\sqrt{{\mathrm{\epsilon }}_{0}{\mathrm{a}}_{0}\mathrm{m}}}$
(c) $\frac{\mathrm{e}}{\sqrt{4{\mathrm{\pi \epsilon }}_{0}{\mathrm{a}}_{0}\mathrm{m}}}$         (d) $\frac{\sqrt{4{\mathrm{\pi \epsilon }}_{0}{\mathrm{a}}_{0}\mathrm{m}}}{e}$