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: Physiology
[{'content': 'Universal blood donor will be a blood group of: \nSelect and explain. \nA. Positive, B. Negative, C. AB positive, D. AB negative', 'role': 'user'}, {'content': 'Answer: B \nExplanation: None', 'role': 'assistant'}]