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
Which of the following is true about fourth heart sound `S4' 
Select and explain. 
A. Can be heard by the unaided ear, B. Frequency is greater than 20 Hz, C. Heard during ventricular filling phase, D. Heard during ventricular ejection phase
C: Heard during ventricular filling phase
Answer: C: Heard during ventricular filling phase 
Explanation: Fourth heart sound occurs due to the late rapid filling phase of the ventricle, which occurs due to atrial systole. Since its frequency is usually less than 20Hz, it is not heard by an unaided ear.