This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Water, Subject name: Social & Preventive Medicine
Softening is recommended when hardeness of water is more than? 
Select and explain. 
A. 50 mg/litre, B. 75 mg/litre, C. 100 mg/litre, D. 150 mg/litre
D: 150 mg/litre
Answer: D: 150 mg/litre 
Explanation: ANSWER: (D) 150 mg/litreREF: Park 20th ed p. 636Indirect repeat from primary DNB CET December 2011Drinking water should be moderately hard. Softening of drinking water Is recommended when the hardness exceeds 3 mEq/L (150 mg/litre)ClassificationLevel of hardness (mEq/L)Soft water< 1 (< 50 mg/1)Moderately hard1-3 (50-150 mgd)Hard water3-6 (150-300 mg/1)Very hard water>6 (> 300 mg/1)