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 ejection phase, D. None of the above
D: None of the above
Answer: D: None of the above 
Explanation: None