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: Surgery
[{'content': 'The principal action of ammonia in syncope is as a: \nSelect and explain. \nA. Vasomotor stimulant, B. Respiratory stimulant, C. Vagal stimulant, D. Inhibitor of vasomotor tone', 'role': 'user'}, {'content': 'Answer: B \nExplanation: None', 'role': 'assistant'}]