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: Surgery
[{'content': 'The optimum length of screw for fixation of plates in mandible: \nSelect and explain. \nA. 3 mm, B. 4 mm, C. 5 mm, D. 6 mm', 'role': 'user'}, {'content': 'Answer: B \nExplanation: Usually the thickness of buccal cortex is around 3.5 mm. A 4 mm length screw is adequate for proper screw-bone contact.', 'role': 'assistant'}]