Exactly 1.5 g of a fuel is burned under conditions of constant pressure

Exactly 1.5 g of a fuel is burned under conditions of constant pressure and then again under conditions of constant volume.In measurement A the reaction produces 25.9 kJ of heat and in measurement B it produces 23.3 kJ of heat. Which measurement ( A or B) corresponds to conditions of constant pressure? Which one corresponds to conditions of constant volume? Explain.