The vapour pressure of pure liquids A and B are 450 and 700 mm Hg respectively, at 350 K.  If the total vapour pressure is 600 mm Hg then the composition of the vapour phase are:

1. X_A=0.30; X_B=0.70
2. X_A=0.40; X_B=0.60
3. X_A=0.20; X_B=0.90
4. X_A=0.50; X_B=0.50