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
The term "Secular Trend" implies 
Select and explain. 
A. Being liberal minded in diagnosing, B. Cold professional attitude, C. Changes in occurrence over a long period of time, D. Migration from rural areas
C: Changes in occurrence over a long period of time
Answer: C: Changes in occurrence over a long period of time 
Explanation: None