A chemist is studying a reaction in which a diatomic gas, Gas X, reacts with another diatomic gas, Gas Y, to form a gaseous compound XY. The reaction is exothermic and proceeds with a decrease in the total number of gas molecules. During the reaction, it is observed that the volume of the gaseous system remains constant, and the pressure decreases. Which of the following statements is true about this reaction at constant temperature? A. The decrease in pressure indicates an increase in the total number of gas molecules. B. The reaction shifts towards the formation of more XY according to Le Chatelier's Principle. C. The reaction violates the law of conservation of mass. D. The decrease in pressure is due to the exothermic nature of the reaction.