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': 'Most common serotype of listeria causing infections: \nSelect and explain. \nA. 1/2a, B. 1/3a, C. 1/3b, D. 1/2c', 'role': 'user'}, {'content': "Answer: A \nExplanation: Ans. is. 'a' i. e., 1/2a", 'role': 'assistant'}]