Concept:Protozoans live in water and remove nitrogenous waste as ammonia.Explanation:Ammonia is very toxic, but it dissolves easily in water.Because protozoans are aquatic, they can get rid of ammonia quickly by diffusion through their body surface.This process requires very little energy and water is plentiful around them.Therefore, ammonia is the main nitrogenous excretory product in protozoans.Answer:C. ammonia