Question 3306 Deck General

The correct answer is D) filling open-circuit breathing systems. Grade D compressed air standards apply to compressed air used for filling open-circuit breathing systems, such as those used in SCUBA diving or other underwater applications. These standards ensure the air quality is safe for human respiration, with limits on contaminants and oxygen content. The other answer choices are not directly related to the Grade D air standard, which is specifically for breathing air systems. Air used for driving pumps, accumulators, or other non-breathing applications would not need to meet this stringent respiratory air quality requirement.