What does the crew mask supply at cabin altitudes above 35,000 ft?

In cabin altitudes above 35,000 feet, the crew mask supplies only pure oxygen because at these altitudes, the atmospheric pressure is significantly lower, and the partial pressure of oxygen is insufficient for normal respiration. Pure oxygen is delivered to ensure that crew members receive adequate oxygen levels to maintain consciousness and function properly. The use of pure oxygen helps prevent hypoxia, a condition that occurs when the body does not get enough oxygen, which can be particularly critical during high-altitude flight operations.

Options describing air with mixed oxygen or oxygen at variable rates do not apply in this scenario, as they would not provide the necessary concentration of oxygen needed at such high altitudes. The choice mentioning humidified air is also irrelevant, as the focus at high altitudes is on delivering adequate oxygen rather than humidity levels.