This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Anaesthesia
[{'content': 'Which of the following systems can be used to produce PEEP \nSelect and explain. \nA. Spring system, B. Ball valve system, C. Pneumatic system, D. All the above', 'role': 'user'}, {'content': 'Answer: D \nExplanation: None', 'role': 'assistant'}]