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
PR interval in ECG denotes 
Select and explain. 
A. Isovolumetric contraction of ventricle, B. Insovolumetric relaxation of heart, C. Atrial contraction, D. None
C: Atrial contraction
Answer: C: Atrial contraction 
Explanation: 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