The energy of an electron revolving in the $n^{th}$ Bohr's orbit of an atom is given by the expression

• A
  $E_n = - \frac{2\pi^2 m^4 e^2 z^2}{n^2 h^2}$
• B
  $E_n = - \frac{2\pi^2 m e^2 z^2}{n^2 h^2}$
• C
  $E_n = - \frac{2\pi^2 m e^4 z^2}{n^2 h^2}$
• D
  $E_n = - \frac{2\pi m^2 e^2 z^4}{n^2 h^2}$