If $p^{\circ}$ and $p$ are the vapour pressure of the pure solvent and solution,and $n_{1}$ and $n_{2}$ are the moles of solute and solvent respectively in the solution,then the correct relation between $p$ and $p^{\circ}$ is:

• A
  $p^{\circ} = p \left[ \frac{n_{1}}{n_{1} + n_{2}} \right]$
• B
  $p^{\circ} = p \left[ \frac{n_{2}}{n_{1} + n_{2}} \right]$
• C
  $p = p^{\circ} \left[ \frac{n_{2}}{n_{1} + n_{2}} \right]$
• D
  $p = p^{\circ} \left[ \frac{n_{1}}{n_{1} + n_{2}} \right]$