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 heart sound is produced by the: 
Select and explain. 
A. Closure of the aortic and pulmonary valves, B. Opening of the aortic and pulmonary valves, C. Closure of the mitral and tricuspid valves, D. Opening of the mitral and tricuspid valves
C: Closure of the mitral and tricuspid valves
Answer: C: Closure of the mitral and tricuspid valves 
Explanation: None