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
Which of the following systems can be used to produce PEEP 
Select and explain. 
A. Spring system, B. Ball valve system, C. Pneumatic system, D. All the above
D: All the above
Answer: D: All the above 
Explanation: None