In the following diagram closure of Aoic valves if indicated by alphabet
High-Yield Explanation
At the end of DE segment which denotes slow ejection phase, ventricular systole ends and intraventricular pressure drops to less than the pressure in the aoa, at this point aoic valves close to prevent the backflow of blood into the left ventricle from aoa, this also produces S2 Ref : Ganong 25th ed/page 540