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
True about typhoid is - 
Select and explain. 
A. 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
D: Vi polysaccharide of bacterial cell used for vaccination
Answer: D: Vi polysaccharide of bacterial cell used for vaccination 
Explanation: Ans. is 'd' i.e., Vi polysaccharide of bacterial cell used for vaccination