What is the specific humidity? A. the ratio of the mass of water vapour to the mass of the total mixture of air and water vapour B. the ratio of the mass of dry air to the mass of the total mixture of air and water vapour C. the ratio of the mass of dry air to the mass of water vapour in a mixture of air and water vapour D. the ratio of the mass of water vapour to the mass of dry air in a mixture of air and water vapour