Why can moist air be considered lighter than dry air?

Correct answer: Because water vapour has lower molecular weight than average dry air

Direct answer: A is correct. Air is a mixture of gases, and its density depends partly on the average molecular mass of the mixture. Water vapour has a molecular mass of about 18, while the average molecular mass of dry air is about 29. When water vapour enters an otherwise similar volume of air, it replaces some molecules such as nitrogen and oxygen, which are heavier on average. Thus the moist mixture can be less dense, especially when temperature and pressure are kept comparable. A does not mean that every parcel of moist air is automatically lighter under every possible condition; temperature and pressure also affect density. B is wrong because water vapour is the gaseous form of water, not a solid. C is wrong because moist air still contains nitrogen, oxygen, and other gases. D is wrong because gravity acts on both dry and moist air. Memory cue: lighter water-vapour molecules replace heavier dry-air molecules.