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': 'What do you understand by the term secular trend ? \nSelect and explain. \nA. Long term changes, B. Sho long term changes, C. Seasonal changes, D. Periodical changes', 'role': 'user'}, {'content': "Answer: A \nExplanation: Ans. is 'a' i.e., Long term changes", 'role': 'assistant'}]