Two identical containers kept at the same temperature carry the same gas with different molecular mean free paths $l_1$ and $l_2$. The gas from the first container is completely transferred into the second at the same temperature. What is the molecular mean free path of the resultant mixture in the second container?

• A

  $\sqrt{l_1 l_2}$
• B

  $l_1+l_2$
• Correct answer
  C

  $\frac{l_1 l_2}{l_1+l_2}$
• D

  $\sqrt{l_1^2+l_2^2}$

Hence, the correct answer is option: 'C'.