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: Pathology
[{'content': 'Green stains which occur frequently in children are due to: \nSelect and explain. \nA. Enamel deficiency, B. Dentin deficiency, C. Material alba, D. Chromogenic bacteria', 'role': 'user'}, {'content': 'Answer: D \nExplanation: None', 'role': 'assistant'}]