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
The frequency of first hea sound is 
Select and explain. 
A. 10-15 Hz, B. 20-25 Hz, C. 25-35 Hz, D. 50 Hz
C: 25-35 Hz
Answer: C: 25-35 Hz 
Explanation: 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