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
Most popular theory is the 
Select and explain. 
A. Hydrodynamic theory, B. Direct nerve stimulation, C. Transduction Theory, D. All are equally popular
A: Hydrodynamic theory
Answer: A: Hydrodynamic theory 
Explanation: None