This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Thorax, Subject name: Anatomy
[{'content': 'The arrow marked structure drains into: \nSelect and explain. \nA. Right atrium, B. Left atrium, C. Right ventricle, D. Left ventricle', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Ans. A. Right atriumThe arrow marked structure is coronary sinus and it drains into right atrium.', 'role': 'assistant'}]