<prompt> Which is the most common agent employed in external bleaching?Sodium perborateHydrochloric acidCarbamide peroxideSodium hypochlorite  <response>: 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 