Concept:Downward displacement of air is used only for gases less dense than air.Explanation:In this method, the gas rises and pushes the heavier air downward.Oxygen is heavier than air because its density is about 1.43g/L, while air is about 1.29g/L.Since O2 is denser than air, it cannot rise to displace the air downward.Therefore, oxygen cannot be collected by downward displacement of air.Oxygen is normally collected over water because it is only slightly soluble in water.Answer:A. The density of oxygen is greater than that of air.