Which one is correct regarding electrocardiograph (ECG)?

The P-wave represents the electrical excitation (or depolarisation) of the atria, which leads to the contraction of both the atria. The QRS complex represents the depolarisation of the ventricles, which initiates the ventricular contraction. The Twave represents the return of the ventricles from excited to normal state (repolarisation). So, by counting the number of QRS complexes that occur in a given time period, one can determine the heart beat rate of an individual.