If $n$ is the number density and $d$ is the diameter of the molecule, then the average distance covered by a molecule between two successive collisions (i.e., mean free path) is represented by:

• A
  $\frac{1}{\sqrt{2} n \pi d^2}$
• B
  $\sqrt{2} n \pi d^2$
• C
  $\frac{1}{\sqrt{2} n \pi d}$
• D
  $\frac{1}{\sqrt{2} n^2 \pi^2 d^2}$