Answer:
A. the blood pressure in the heart chamber from which blood is flowing is higher than the blood pressure in the chamber to which the blood is supposed to flow.
Explanation:
A heart valve opens or shuts on both sides with varying blood pressure. As the heart muscle contracts and relaxes, the valves open and shut. This lets blood flow into the ventricles and atria at alternate times.
When the left ventricle relaxes, the aortic valve closes and the mitral valve opens. This lets blood flow from the left atrium into the left ventricle.
