In a hydrogen like atom electron makes transition from an energy level with quantum number $n$ to another with quantum number $(n-1)$. If $n\gg 1$, the frequency of radiation emitted is proportional to:

1. A
   \frac{1}{n^2}
2. B
   \frac{1}{n^{3/2}}
3. C
   \frac{1}{n^3}
4. D
   \frac{1}{n}