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': 'The first hea sound is due to: September 2005 \nSelect and explain. \nA. AV valves closure, B. Closing of aoic and pulmonary valves, C. Opening of AV valves, D. Opening of aoic and pulmonary valves', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Ans. A: AV valves closure', 'role': 'assistant'}]