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
The hardness of drinking water should be? 
Select and explain. 
A. >3, B. <1, C. 3-Jan, D. >6
C: 3-Jan
Answer: C: 3-Jan 
Explanation: ANSWER: (C) 1-3REF: Park 20th ed p. 636Drinking water should be moderately hard. Softening of drinking water Is recommended when the hardness exceeds 3 mEq/LClassificationLevel of hardness ( mEq/L)Soft water< 1 ( < 50 mg/l)Moderately hard1-3 ( 50-150 mg/l )Hard water3-6 (150-300 mg/l)Very hard water> 6 ( > 300 mg/l)