This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Cardiovascular system, Subject name: Physiology
[{'content': 'S3 is due to \nSelect and explain. \nA. Rapid gushing of blood into ventricle, B. Atrial contraction, C. AV valve closure, D. Semilunar valve closure', 'role': 'user'}, {'content': 'Answer: A \nExplanation: S3 is due to the rapid gush of blood in the ventricle Ref: Ganong 25th ed/page 540', 'role': 'assistant'}]