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: Unknown
[{'content': 'Unit of ecological study is: \nSelect and explain. \nA. Population, B. Individual, C. Healthy people, D. Patient', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Ref. Foundations of Community medicine. Page. 107\n\xa0\nIn ecological studies the unit of observation is the population or community.\n\nDisease rates and exposures are measured in each of a series of populations and their relation is examined.\nOften the information about disease and exposure is abstracted from published statistics and therefore does not require expensive or time', 'role': 'assistant'}]