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
Sentinel surveillance; TRUE is: 
Select and explain. 
A. Data is itself reported to the health system, B. Active collection of data, C. Employed to identify the missing case, D. All of the above
C: Employed to identify the missing case
Answer: C: Employed to identify the missing case 
Explanation: Surveillance: 

Ongoing systematic collection, analysis & interpretation of data and use of this information to take action for prevention and control of disease. 
Used for purpose of 'health planning'. 

Types of Surveillance: