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
Which of the following is not another name for the vital layer in slow sand filter? 
Select and explain. 
A. Schmutzdecke, B. Zoogleal layer, C. Biological layer, D. Essential layer
D: Essential layer
Answer: D: Essential layer 
Explanation: Vital layer is also known as Schmutzdecke, zoogleal layer, biological layer. It is also known as the hea of the slow sand filter. Ref: Park 21st edition page: 659.