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: Dental
[{'content': 'Most popular theory is the \nSelect and explain. \nA. Hydrodynamic theory, B. Direct nerve stimulation, C. Transduction Theory, D. All are equally popular', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]