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
All of the following hemostatic agents are used as perforation repair, except? 
Select and explain. 
A. Collagen, B. CaOH2, C. Ferric sulphate, D. Fridge dried bone
C: Ferric sulphate
Answer: C: Ferric sulphate 
Explanation: None