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 is the gas which produces systemic toxicity without causing local irritation? \nSelect and explain. \nA. Ammonia, B. Carbon monoxide, C. Hydrocyanic acid, D. Sulfur dioxide', 'role': 'user'}, {'content': "Answer: B \nExplanation: Carbon monoxide is a colourless odourless and tasteless gas. Headache and nausea may be an initial sign of poisoning. Since the PO2 is normal the respiratory centres are also not stimulated. The victim is simply asphyxiated even without him knowing it. Ref: Harrison's Principles of Internal Medicine, 18th Edition, Chapter 52", 'role': 'assistant'}]