For a dilute solution containing 2.5 g of a non- volatile non- electrolyte solute in 100 g of water, the elevationin boiling point at 1 atm pressure is 2ºC. Assuming concentration of solute is much lower than the concentration of solvent, the vapour pressure (mm of Hg) of the solution is (take Kb= 0.76 K kg mol)
(A) 724 (B) 740 (C) 736 (D)718

sir,I solved this problem and got the answer(A) but my doubt is that, we know that boiling point is that temperature at which vapour pressure of the solution =vapour pressure of the external pressure which is 1atm in this question so why cannot 1atm be the answer for vapour pressure of the solution BY THE DEFINITION OF BOILING POINT.