An ideal gas at pressure $P$ and temperature $T$ is enclosed in a vessel of volume $V$. Some gas leaks through a hole from the vessel and the pressure of the enclosed gas falls to $P'$. Assuming that the temperature of the gas remains constant during the leakage, the number of moles of the gas that have leaked is

• A
  $\frac{P' V}{RT}$
• B
  $\frac{V}{RT} (P - P')$
• C
  $\frac{2V}{RT} (P + P')$
• D
  $\frac{V}{RT} (P + P')$