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': 'Purest water in nature is \nSelect and explain. \nA. Rain water, B. River water, C. Deep well, D. Impounding reservoirs', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Rain : Is the prime source of all water and is the ‘purest form of water in nature.’\nChemically, it is very soft water : contains traces (0.0005%) of solids Gibraltar depends on rain water as a source of supply.', 'role': 'assistant'}]