Concept:The setup demonstrates the movement of gas particles from a region of higher concentration to one of lower concentration.Explanation:In the typical diffusion experiment, cotton wool soaked in ammonia and concentrated hydrochloric acid are placed at opposite ends of a glass tube. The gases travel along the tube and meet to form a white precipitate/ring of ammonium chloride. The formation and position of this ring show that gases diffuse at different rates through air.Answer:A. diffusion
