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 green stains frequently seen on children's teeth are
caused by 
Select and explain. 
A. Materia alba, B. Enamel defects, C. Dentinal defects, D. Chromogenic bacteria
D: Chromogenic bacteria
Answer: D: Chromogenic bacteria 
Explanation: None