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: Dental
[{'content': '"Loss of Head" is: \nSelect and explain. \nA. Death of a head of a family, B. Increased resistance to filtration in a sand filter (related to filtration capacity), C. Death of a head in village panchayat, D. Death of a school teacher who is also a healthy worker of a school', 'role': 'user'}, {'content': 'Answer: B \nExplanation: RAPID SAND OR MECHANICAL FILTERS\nFiltration removes the remaining alum\xa0not removed by sedimentation. As filtration proceeds, the suspended impurities and bacteria clog the filters resulting in their reduced efficiency called "loss of head”.\nBackwashing\nRapid sand filters need frequent washing daily or weekly, depending upon the loss of head. Washing is accomplished by reversing the flow of water through the sand bed, which is called "backwashing". It helps in dislodging the impurities and cleaning up the sand bed.\xa0\nEssentials of preventive and community dentistry\xa0\nSoben Peter\xa0\n5th edition', 'role': 'assistant'}]