The rate of diffusion of a gas $A$ is $\sqrt{5}$ times more than that of gas $B$. If the molar mass of $A$ is $x \ g \ mol^{-1}$, the molar mass of $B \ (\text{in } g \ mol^{-1})$ is (in $x$)

• A
  $4$
• B
  $5$
• C
  $16$
• D
  $25$