Pick up the correct statement from the following :

• Absolute humidity at a given temperature is equal to weight of moisture present in a unit volume
• Relative humidity is the ratio of actual vapour pressure and saturation vapour pressure at the same temperature
• Relative humidity is the ratio of the weight of the vapours present per unit volume to the weight of vapours which could be contained at the same temperature when fully saturated
• Humidity can be measured by psychrometer
• All the above.