A non-volatile solute (A) is dissolved in a volatile solvent (B). The vapour pressure of the resultant solution is $P_S$. The vapour pressure of pure solvent is $P_B$. If $X_B$ is the mole fraction of the solvent which of the following is correct :

Question

( A ) $P^{\circ}_S = X_B. P_B^{^{\circ}}$
( B ) $P_S = X_A.P_B^{^{\circ}}$
( C ) $P^{^{\circ}}_B = P_S. X_A$
( D ) $P^{^{\circ}}_B = P_S. X_B$