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
Retention appliance should be 
Select and explain. 
A. Retentive only, B. Passive only, C. Passive + retentive, D. Active + retentive
C: Passive + retentive
Answer: C: Passive + retentive 
Explanation: None