Which one of the following statements is correct?

1. A. The tricuspid and the bicuspid valves open due to the pressure exerted by the simultaneous contraction of the atria
2. B. Blood moves freely from atrium to the ventricle during joint diastole.
3. C. Increased ventricular pressure causes closing of the semilunar valves.
4. D. The atrio-ventricular node (AVN) generates an action potential to stimulate atrial contraction

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