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
Gingivitis is initiated most frequently by: 
Select and explain. 
A. Pregnancy, B. Malocclusion, C. Vitamin deficiency, D. Local irritating factors
D: Local irritating factors
Answer: D: Local irritating factors 
Explanation: None