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
[{'content': 'Softening is recommended when hardness of water is more than: \nSelect and explain. \nA. 50mg/litre, B. 75mg/litre, C. 100mg/litre, D. 150mg/litre', 'role': 'user'}, {'content': 'Answer: D \nExplanation: Softening is recommended when hardness of water is more than 150 mg/L (> 3mEq/ L). Softening of water is done by Temporary hardness Permanent hardness Boiling Lime NaCo3 Permutit NaC03 Base exchange', 'role': 'assistant'}]