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
An inexpensive and efficient method of sewage disposal for a small community is - 
Select and explain. 
A. River outfall, B. Oxidation pond, C. Trickling filter, D. Activated sludge
B: Oxidation pond
Answer: B: Oxidation pond 
Explanation: Oxidation ponds: It is a cheap method of sewage treatment Aka waste stabilization pond, redox pond, sewage lagoons, etc. It is an open shallow pool 1 to 1.5 m deep with an inlet and outlet there must be the presence of algae, ceain bacterias, and sunlight. Organic matter oxidized by bacteria to simple compounds such as CO2, ammonia, and water. Algae with the help of sunlight utilize CO2 and other inorganic compounds. Park's Textbook of Preventive and Social Medicine, 25th Edition, Pg 825