The mean free path for a gas, with molecular diamter d and number density n can be expressed as :

• A

  $\frac{1}{\sqrt{2}n\pi d^2}$

• B

  $\frac{1}{\sqrt{2}{n}^2\pi d^2}$

• C

  $\frac{1}{\sqrt{2}{n}^2{\pi }^2{d}^2}$

• D

  $\frac{1}{\sqrt{2}n\pi d}$