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
The occurrence of keratinization on the human gingiva is 
Select and explain. 
A. 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
D: Lacking in the gingival crevice
Answer: D: Lacking in the gingival crevice 
Explanation: None