Concept:Dried oxygen must be collected in a way that does not add moisture to the gas.Explanation:Oxygen is commonly collected over water because it is insoluble in water.However, to obtain dried oxygen, water must be avoided.Mercury is a liquid metal that contains no water, so dry oxygen can be collected over mercury without becoming moist.The other options involve aqueous acids or a solid drying agent, not suitable for collecting a dry gas.Answer:B. mercury