单项选择题 If an air compressor is used to supply air primarily to the combustion control system and other pneumatic controllers, the entire system is known as the （）.

单项选择题 When it is necessary to open a refrigeration system for repairs and the system will be open for more than a few minutes, the open ends of the system should be （）.

单项选择题 If an air compressor is used to supply compressed air to outlets throughout the engine room and on deck of a vessel, the system is known as the （）.