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
Monitoring of rate of occurrence of specific conditions to assess the stability or change in health levels of a population is known as: 
Select and explain. 
A. Epidemiological surveillance, B. Active surveillance, C. Passive surveillance, D. Sentinel surveillance
D: Sentinel surveillance
Answer: D: Sentinel surveillance 
Explanation: None