For the diffusion of a gas at pressure $P$,the rate of diffusion is expressed by :

• A
  $r \propto \frac{1}{\sqrt{M}}$
• B
  $r = \frac{P}{M}$
• C
  $r \propto \frac{M}{\sqrt{P}}$
• D
  $r \propto \frac{P}{\sqrt{M}}$