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 widal test \nSelect and explain. \nA. Anti-O antibody persists longer, B. O antigen of S paratyphi is used., C. H-antigen is most immunogenic., D. Felix tube is used for H agglutination.', 'role': 'user'}, {'content': 'Answer: C \nExplanation: None', 'role': 'assistant'}]