The vapour pressure of pure liquids A and B are 450 and 700 mm Hg respectively, at 350 K. If total vapour pressure is 600 mm Hg then the composition of the vapour phase are-
1. XA=0.30; XB=0.70
2. XA=0.40; XB=0.60
3. XA=0.20; XB=0.90
4. XA=0.50; XB=0.50