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': 'Luebke ochsenbein flap is done in \nSelect and explain. \nA. When marginal gingiva need to preserved, B. When Prosthetic crowns are present, C. To avoid non-pathogenic dehiscence, D. All the above', 'role': 'user'}, {'content': 'Answer: D \nExplanation: None', 'role': 'assistant'}]