This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: All India exam, Subject name: Social & Preventive Medicine
Minimum depth for the lining of a sanitary well is? 
Select and explain. 
A. 10 feet, B. 20 feet, C. 35 feet, D. 50 feet
B: 20 feet
Answer: B: 20 feet 
Explanation: The lining of a sanitary well should go down at least 20 feet to ensure that water enters from the bottom and not from the sides of the well. The lining should extend upwards for about 2-3 feet above the ground. Ref: Park's Textbook of Preventive and Social Medicine, 20th edition, p620