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
All included in WHO criteria for water quality except 
Select and explain. 
A. Hardness of water below 3000 mg/l, B. Lead below 0.01 mg/l, C. Nitrates <3 mg/l, D. Zero infections micro - organisms
C: Nitrates <3 mg/l
Answer: C: Nitrates <3 mg/l 
Explanation: Nitrates < 45 mg/l
Nitrites <3 mg /l