This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Electrocardiography, Subject name: Medicine
Wave (Arrow) shown in the Phtograph is due to 
Select and explain. 
A. Atrial depolarization, B. Atrial repolarization, C. Ventricular depolarization, D. Ventricular repolarization
A: Atrial depolarization
Answer: A: Atrial depolarization 
Explanation: Atrial depolarization (Wave shown: p-wave in ECG)