This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: All India exam, Subject name: Social & Preventive Medicine
[{'content': 'Study unit of ecological study is \nSelect and explain. \nA. Population, B. Patient, C. Community, D. Case', 'role': 'user'}, {'content': 'Answer: A \nExplanation: In ecological studies the unit of observation is the population or community. * Disease rates and exposures are measured in each of a series of populations and their relation is examined. * Often the information about disease and exposure is abstracted from published statistics and therefore does not require expensive or time', 'role': 'assistant'}]