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: Anatomy
[{'content': 'The base of the heart is mainly formed by the - \nSelect and explain. \nA. Right atrium, B. Left atrium, C. Left ventricle, D. Right ventricle', 'role': 'user'}, {'content': 'Answer: B \nExplanation: Base of heart is formed by left and right atria. But the major part of it is by left atrium.', 'role': 'assistant'}]