_The relative humidity is defined as$?

• A. The mass of water vapour present in 1 m3 of dry air
• B. The mass of water vapour present in 1 kg of dry air
• C. The ratio of the actual mass of water vapour in a unit mass of dry air to the mass of water vapour in the same mass of dry air when it is saturated at the same temperature and pressure
• D. The ratio of actual mass of water vapour in a given volume of moist air to the mass of water vapour in the same volume of saturated air at the same temperature and pressure