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
The first hea sound is due to: September 2005 
Select and explain. 
A. AV valves closure, B. Closing of aoic and pulmonary valves, C. Opening of AV valves, D. Opening of aoic and pulmonary valves
A: AV valves closure
Answer: A: AV valves closure 
Explanation: Ans. A: AV valves closure