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': "Hepatitis B surface antigen in a patient's serum indicates\nthat the patient is: \nSelect and explain. \nA. Recovered, B. Potentially infectious, C. Now an acceptable blood donor, D. Immune to subsequent exacerbations of the disease", 'role': 'user'}, {'content': 'Answer: B \nExplanation: None', 'role': 'assistant'}]