A. Coronary arteries are filled with blood during atrial contraction: Coronary arteries are primarily perfused during diastole, not atrial contraction. During systole, contraction of the ventricles compresses the coronary vessels, reducing blood flow. B. Coronary circulation occurs only during systole: Coronary blood flow mostly occurs during diastole, because the myocardial contraction during systole compresses the coronary vessels, especially in the left ventricle. C. Coronary veins return deoxygenated blood to the right atrium: Coronary veins collect deoxygenated blood from the myocardium and empty it into the right atrium via the coronary sinus, completing the coronary circulation. D. Coronary arteries supply deoxygenated blood to the heart muscle: Coronary arteries supply oxygenated blood to the heart muscle, not deoxygenated blood.