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
Which is the most common agent employed in external bleaching? 
Select and explain. 
A. Sodium perborate, B. Hydrochloric acid, C. Carbamide peroxide, D. Sodium hypochlorite
C: Carbamide peroxide
Answer: C: Carbamide peroxide 
Explanation: The most common bleaching agent for at-home bleaching is 10% to 22% carbamide peroxide with an effective yield of 4% to 7.5% hydrogen peroxide.
Ref: Cohen pathways of Pulp 11th ed page no  e107