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': 'The occurrence of keratinization on the human gingiva is \nSelect and explain. \nA. Dependent upon the functional stimulation the attached gingival receives, B. A reflection of inflammation in the underlying gingival connective tissue, C. A variable factor directly related to age, D. Lacking in the gingival crevice', 'role': 'user'}, {'content': 'Answer: D \nExplanation: None', 'role': 'assistant'}]