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
Frequency of 1st hea sound 
Select and explain. 
A. 10-15 Hz, B. 20-25 Hz, C. 25-45 Hz, D. 50 Hz
C: 25-45 Hz
Answer: C: 25-45 Hz 
Explanation: The first sound has a duration of about 0.15 s and a frequency of 25-45 Hz. It is soft when the hea rate is low because the ventricles are well filled with blood and the leaflets of the AV valves float together before systole.Ref: Ganong&;s Review of medical physiology 25th edition. Page: 542