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: Dental
[{'content': 'A split thickness flap is indicated when: \nSelect and explain. \nA. Osseous craters are present, B. Fenestrations and Dehiscence are suspected, C. Buttressing bone formation is present, D. Three walled osseous defects are encountered', 'role': 'user'}, {'content': 'Answer: B \nExplanation: None', 'role': 'assistant'}]