This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Cardiovascular system, Subject name: Physiology
[{'content': 'The frequency of first hea sound is \nSelect and explain. \nA. 10-15 Hz, B. 20-25 Hz, C. 25-35 Hz, D. 50 Hz', 'role': 'user'}, {'content': 'Answer: C \nExplanation: S1 is caused due to the closure of AV valves, it is long and soft with duration of 0.15s Ref: Ganong 25th ed/page 541', 'role': 'assistant'}]