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
Past contamination of water is indicated by – 
Select and explain. 
A. Free and saline ammonia, B. Dissolved H2S, C. Nitrites, D. Nitrates
D: Nitrates
Answer: D: Nitrates 
Explanation: None