This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: All India exam, Subject name: Microbiology
[{'content': 'Microlaryngoscopy is usually done with an objective lens with focal length of \nSelect and explain. \nA. 100 mm, B. 200 mm, C. 300 mm, D. 400 mm', 'role': 'user'}, {'content': 'Answer: D \nExplanation: Surgical microscopes have the ability to fine focus with focal lengths that may vary from the standard microlaryngeal surgical focal length of 400 mm.', 'role': 'assistant'}]