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: Social & Preventive Medicine
[{'content': 'Schmutzdecke refers to the following \nSelect and explain. \nA. Suspended matter in drinking water, B. Algae in drinking water, C. Algae, plankton, diatoms and bacteria on surface of sand bed filter, D. Alum flocculate on surface of sand bed filter', 'role': 'user'}, {'content': 'Answer: C \nExplanation: None', 'role': 'assistant'}]