This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Environment and health, Subject name: Social & Preventive Medicine
Hardness of water is expressed as milli-equivalent per litre (mEq/L), one unit of which is equivalent to which one of the following - 
Select and explain. 
A. 20 mg CaCO3 (20 ppm), B. 30 mg CaCO3 (30 ppm), C. 40 mg CaCO3 (40 ppm), D. 50 mg CaCO3 (50 ppm)
D: 50 mg CaCO3 (50 ppm)
Answer: D: 50 mg CaCO3 (50 ppm) 
Explanation: Hardness of water us expressed in milliequivalents/L. One mEq/L is equal to 50mg of CaCO3(50 ppm) in one litre. Park's Textbook of Preventive and Social Medicine, 25th edition, Page No. 786