This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Social & Preventive Medicine
[{'content': 'In a slow sand filter, the element responsible for yielding bacteria free water is the \nSelect and explain. \nA. Risk of Guinea worm infection, B. Water is more than depth of 15 meter, C. Where no water source in a distance of 1.6 km from community, D. There is excess of Na+, K+, F+ salts', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Level of hardness is expressed in terms of milliequivalents per litre where one milliequivalent is equal to 50mg calcium carbonate in one litre of water.\nThe level of hardness of hard water is equal to 3-6mEq/L or 150-300 mg/L.', 'role': 'assistant'}]