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
Old contamination of water is indicated by 
Select and explain. 
A. Nitrites, B. Nitrates, C. Ammonia, D. Chlorides
B: Nitrates
Answer: B: Nitrates 
Explanation: Ans. is 'b' i.e., Nitrates