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
In the etiology of dental caries, one of the following is
not an environmental factor 
Select and explain. 
A. Saliva, B. Micro organisms, C. Refined carbohydrates, D. Structure of the tooth
D: Structure of the tooth
Answer: D: Structure of the tooth 
Explanation: None