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': 'True regarding secular trends - \nSelect and explain. \nA. Changes in disease pattern in terms of months, B. Changes in disease pattern seasonally, C. Changes in disease pattern over a short period of timer, D. Changes in disease pattern over a long period of time', 'role': 'user'}, {'content': 'Answer: D \nExplanation: None', 'role': 'assistant'}]