This is a graph showing the readout from an electrocardiogram. The graph plot starts as a horizontal line at the center of the image. The plot line then has a small peak, after which it returns to horizontal for a short section,only to then form a second large peak. While the plot line is descending the large peak, it dips below the center of the figure to form an upside-down peak, which is similar in size to the previous large peak. After returning to the base of the upside-down peak, a final small peak occurs before the plot line becomes horizontal again and runs off the page. An arrow is pointing to the first small peak of the graph. The wave with the arrow on the electrocardiogram corresponds to which event? (2 points) Atrial depolarization Atrial repolarization Ventricular depolarization Ventricular repolarization