If $a_0$ is the Bohr radius of the hydrogen atom, what is the de-Broglie wavelength $(\lambda)$ of an electron in the $n^{th}$ orbit of the hydrogen atom?

• A
  $2 \pi n a_0$
• B
  $\frac{2 \pi a_0}{n}$
• C
  $\frac{n a_0}{2 \pi}$
• D
  $\frac{2 \pi n^2}{a_0}$