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
Main function of GTR is/are: 
Select and explain. 
A. Space maintenance, B. Clot stabilization, C. Wound contraction, D. All of the above
D: All of the above
Answer: D: All of the above 
Explanation: None