This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Environment and health, Subject name: Social & Preventive Medicine
[{'content': 'Most impoant layer of a slow sand filter is - \nSelect and explain. \nA. Zoological layer, B. Sand bed, C. Filter system, D. Raw water', 'role': 'user'}, {'content': 'Answer: A \nExplanation: The most impoant layer of a slow sand filter is the vital layer or zoogleal layer or the biological layer. It is composed of thread like algae and other forms of life. It is called so because it removes organic matter, holds back bacteria and oxidises ammoniacal nitrogen and helps in yielding bacteria-free water. (PARK TB 25th edition pg 772)', 'role': 'assistant'}]