In a mixture of liquids $A$ and $B$, if the mole fractions of component $A$ in vapour phase and liquid mixture are $x_1$ and $x_2$ respectively, then the total vapour pressure of liquid mixture is (where $P_{A}^{\circ}$ and $P_{B}^{\circ}$ are the vapour pressures of pure $A$ and $B$)

• A
  $\frac{P_{B}^{\circ} x_1}{x_2}$
• B
  $\frac{P_{B}^{\circ} x_2}{x_1}$
• C
  $\frac{P_{A}^{\circ} x_2}{x_1}$
• D
  $\frac{P_{A}^{\circ} x_1}{x_2}$