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
[{'content': 'Hardness of water is expressed as milli-equivalent per litre (mEq/L), one unit of which is equivalent to which one of the following - \nSelect and explain. \nA. 20 mg CaCO3 (20 ppm), B. 30 mg CaCO3 (30 ppm), C. 40 mg CaCO3 (40 ppm), D. 50 mg CaCO3 (50 ppm)', 'role': 'user'}, {'content': "Answer: D \nExplanation: 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", 'role': 'assistant'}]