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
[{'content': 'Which of the following is true about fourth heart sound S4 \nSelect and explain. \nA. 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', 'role': 'user'}, {'content': 'Answer: D \nExplanation: None', 'role': 'assistant'}]