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: Microbiology
[{'content': 'True about typhoid is - \nSelect and explain. \nA. Incubation period 3-6 weeks, B. Chronic carrier is 10-15%, C. Widal test is specific, D. Vi polysaccharide of bacterial cell used for vaccination', 'role': 'user'}, {'content': "Answer: D \nExplanation: Ans. is 'd' i.e., Vi polysaccharide of bacterial cell used for vaccination", 'role': 'assistant'}]