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: Medicine
[{'content': "The 'a' wave of jugular venous pulse is produced by- \nSelect and explain. \nA. Atrial relaxation, B. Atrial systole, C. Atrial diastole, D. Rapid venous filling", 'role': 'user'}, {'content': 'Answer: B \nExplanation: None', 'role': 'assistant'}]