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': "Gingival extension for a restoration should be: \nSelect and explain. \nA. At the gingival crest, B. At Least 1mm above the alveolar crest, C. At Least 3mm above the alveolar crest, D. Doesn't have any relation with alveolar crest height", 'role': 'user'}, {'content': 'Answer: C \nExplanation: None', 'role': 'assistant'}]