In a standard ECG which one of the following alphabets

In a standard ECG which one of the following alphabets is the correct representation of the respective activity of the human heart?

1. S - start of systole
2. T - end of diastole
3. P - depolarisation of the atria
4. R - repolarisation of ventricles

In a standard ECG, the P-wave is a small upward wave that indicates the depolarisation of the atria. This is caused by the activation of SA node.