This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Cardiac Cycle, Subject name: Physiology
[{'content': "Which of the following is true about fouh hea sound 'S4' \nSelect and explain. \nA. 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", 'role': 'user'}, {'content': 'Answer: C \nExplanation: 4th hea sound cannot be heard even by a stethoscope since its frequency is below the audible limit (20 Hz). It is produced during last rapid filling of ventricles, which occurs due to atrial systole. Hence, it is also called "atrial sound". S1: Closure of A.V. valves (25-45 Hz) and S2: closure of semilunar valves (50 Hz). They can be heard by stethoscope.', 'role': 'assistant'}]