p_A and p_B are the vapour pressure of pure liquid components, A and B, respectively of an ideal binary solution.
If X_A represents the mole fraction of component A, the total pressure of the solution will be:

1. p_A + X_A (p_B-p_A)
2. p_A + X_A (p_A-p_B)
3. p_B + X_A (p_B-p_A)
4. p_B + X_A (p_A-p_B)