the vapour density of a gas may be defined as

• A.the mass of a unit volume of the gas compared to an equal volume of water vapour
• B.the mass of a unit volume of the gas compared to an equal volume of hydrogen
• C.the mass of a unit volume of the gas compared to an equal volume of oxygen
• D.the mass of a unit volume of the gas minus the vapour pressure of water
• E.two times the relative molecular mass of the gas