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': 'Coronary sinus is guarded by \nSelect and explain. \nA. Crista terminalis, B. Thebesian valve, C. Mitral valve, D. Spiral valve', 'role': 'user'}, {'content': "Answer: B \nExplanation: Opening of coronary sinus is guarded by thebesian valve. BD CHAURASIA'S HUMAN ANATOMY VOLUME 1. 6TH EDITION.Page no -255", 'role': 'assistant'}]