Which statement about oxygen used in aircraft systems is true?

The statement that oxygen used in aircraft systems is 99.5 percent pure and practically water-free is accurate because aviation-grade oxygen must meet stringent purity standards to ensure the safety and efficiency of aircraft operations. This high level of purity is essential to prevent any risks of combustion or adverse reactions that could occur with impurities. Water and other contaminants can lead to freezing or malfunctions in the aircraft's oxygen delivery system, which could endanger the safety of passengers and crew. Therefore, aircraft oxygen systems are designed to provide a reliable source of extremely pure oxygen, minimizing the risk of issues related to moisture or impurities. This is crucial for maintaining a safe cabin atmosphere at high altitudes where aircraft operate, ensuring that the oxygen supply remains effective and reliable in emergency situations.