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 anaesthetic gas is contraindicated in the presence of pneumothorax ? \nSelect and explain. \nA. Halothane, B. Isoflurane, C. Nitrous oxide, D. Chloroform', 'role': 'user'}, {'content': 'Answer: C \nExplanation: None', 'role': 'assistant'}]