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
Hepatitis B surface antigen in a patient's serum indicates
that the patient is: 
Select and explain. 
A. Recovered, B. Potentially infectious, C. Now an acceptable blood donor, D. Immune to subsequent exacerbations of the disease
B: Potentially infectious
Answer: B: Potentially infectious 
Explanation: None