This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Physiology
[{'content': 'First heart sound occurs during the: \nSelect and explain. \nA. Isovolumetric ventricular systole., B. Isovolumetric ventricular diastole., C. Protodisatole., D. Ventricular ejection.', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Isovolumetric ventricular systole is ended with the opening of the seminar valves, due to which first heart sound is produced.', 'role': 'assistant'}]