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
[{'content': 'Materia alba \nSelect and explain. \nA. Lack microorganisms, B. Has an irritating effect on gingiva, C. Has a regular internal pattern, D. Is only salivary protein and food debris', 'role': 'user'}, {'content': 'Answer: B \nExplanation: None', 'role': 'assistant'}]