Suppose an electron is attracted towards the origin by a force $F = \frac{k}{r}$,where $k$ is a constant and $r$ is the distance of the electron from the origin. By applying the Bohr model to this system,the radius of the $n^{th}$ orbital of the electron is found to be $r_n$ and the kinetic energy of the electron is $K_n$. Then which of the following is true?

• A
  $K_n \propto \frac{1}{n}, r_n \propto n^2$
• B
  $K_n \propto \frac{1}{n^2}, r_n \propto n^2$
• C
  $K_n$ is independent of $n, r_n \propto n$
• D
  $K_n \propto \frac{1}{n}, r_n \propto n$