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
Periodontitis is caused by 
Select and explain. 
A. Malnutrition, B. Supra gingival plaque, C. Bio-film, D. Faulty tooth brushing
C: Bio-film
Answer: C: Bio-film 
Explanation: None