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': 'Which is true about Widal reaction - \nSelect and explain. \nA. Antibody to H Ag appears first and persists, B. Antibody to 0 Ag appears first and persists, C. Antibodies to H & 0 Ag appears simultaneously and persists, D. None of the above', 'role': 'user'}, {'content': "Answer: A \nExplanation: Ans. is 'a' i.e., Antibody to 'H' antigen appears first and persists", 'role': 'assistant'}]