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': 'Which of the following full thickness flap is contraindicated for endodontic surgeries: \nSelect and explain. \nA. Triangular flap., B. Rectangular flap., C. Trapezoidal flap., D. Envelope flap.', 'role': 'user'}, {'content': 'Answer: C \nExplanation: As collagen fibers and blood vessels run parallel to the root, In trapezoidal flap more number if collagen fibres and blood vessels are cut in turn it delays healing.', 'role': 'assistant'}]