Final answer:
ST-segment and T-wave changes on an electrocardiogram (ECG) occur during ventricular repolarization. The ventricles are repolarizing and preparing for the next cycle, and this electrical activity is represented by the T wave. Atrial repolarization is present but not visible on the ECG as it is masked by the QRS complex.
Step-by-step explanation:
ST-segment and T-wave changes occur during ventricular repolarization. The ST-segment corresponds to the period when the ventricles are depolarized and is relatively isoelectric, which means no net electrical activity is happening. The T wave represents ventricular repolarization, when the ventricles are getting ready to start the cycle over again. It is important to note that atrial repolarization also occurs, but it is masked by the much larger QRS complex that represents ventricular depolarization.
When viewing an electrocardiogram (ECG), there are five prominent points: the P wave, the QRS complex, and the T wave. The P wave represents atrial depolarization, which is when the atria contract. The repolarization of the atria is not visible on the ECG because it happens at the same time as the ventricular depolarization, and thus is obscured by the QRS complex.
In conclusion, options B 'During ventricular repolarization' and D 'During ventricular repolarization' correctly describe when ST-segment and T-wave changes occur; during ventricular repolarization. Atrial repolarization is not seen on ECG due to it being masked by the larger event of ventricular depolarization.