The qrs complex is associated with the qrs complex is associated with depolarization of the ventricular septum depolarization of the ventricles repolarization of the ventricles depolarization of the atria

In a typical ECG (Electrocardiogram) the first wave obtained is P wave; The P wave represents the depolarization of the atria. After this, the QRS complex is formed. The QRS wave represents the depolarization of the ventricles.  During the depolarization phase, the ventricles contracts and pushes the blood into the atria. Hence, the correct answer is depolarization of the ventricles.

i'd say that the moon rock is about 2.2 billion years old.