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 $P^{\prime}$. Assuming that the temperature of the gas remains constant during the leakage,the number of moles of the gas that have leaked is

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