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': 'The term "Secular Trend" implies \nSelect and explain. \nA. Being liberal minded in diagnosing, B. Cold professional attitude, C. Changes in occurrence over a long period of time, D. Migration from rural areas', 'role': 'user'}, {'content': 'Answer: C \nExplanation: None', 'role': 'assistant'}]