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: Physiology
[{'content': 'Speed of conduction is fastest in \nSelect and explain. \nA. AVN, B. SAN, C. Bundle of His, D. Purkinje system', 'role': 'user'}, {'content': 'Answer: D \nExplanation: None', 'role': 'assistant'}]