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': '"Atrial" sound is: \nSelect and explain. \nA. S1, B. S2, C. S3, D. S4', 'role': 'user'}, {'content': 'Answer: D \nExplanation: Fouh hea sound is caused during the last ventricular filling, which is caused by the atrial systole and turbulence blood as it fills into ventricles. Hence, it is referred to as "atrial" sound.', 'role': 'assistant'}]