This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Heart, Circulation, and Blood, Subject name: Physiology
[{'content': 'PR interval in ECG denotes \nSelect and explain. \nA. Isovolumetric contraction of ventricle, B. Insovolumetric relaxation of heart, C. Atrial contraction, D. None', 'role': 'user'}, {'content': 'Answer: C \nExplanation: Ans. (c) Atrial contractionRef: Ganong, 25th ed/p.525PR interval#From the onset of P wave to the beginning of QRS complex#Duration - 0.12 to 0.20 sec#Denotes atrial contraction and Atrioventricular (AV) conduction time', 'role': 'assistant'}]