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: Physiology
[{'content': 'In caissons disease pain in joint is because of \nSelect and explain. \nA. Nitrogen bubble, B. Oxygen bubble, C. Carbon monoxide, D. Air in joint', 'role': 'user'}, {'content': 'Answer: A \nExplanation: In Caisson’s disease, the gas embolism is caused by nitrogen that expands in the tissues and passes to the circulation.', 'role': 'assistant'}]