When wet-bulb depression is large, what is the correct conclusion about evaporation and humidity?

Correct answer: Evaporation is high and relative humidity is low.

The correct answer is D. Wet-bulb depression is the difference between the dry-bulb and wet-bulb temperatures. Water on the wet bulb evaporates and removes heat. When the surrounding air is dry, it can accept more water vapour, so evaporation is rapid and cooling is strong. The wet-bulb temperature then becomes much lower than the dry-bulb temperature, producing a large depression. D correctly links the large depression with vigorous evaporation and low relative humidity. A and C are wrong because air near saturation cannot accept much additional vapour; evaporation is then weak and the temperature difference is small. B is also wrong because nearly stopped evaporation would cause little wet-bulb cooling, not a large difference. The conclusion concerns relative humidity, not necessarily the absolute amount of water vapour alone; warm air can hold more vapour than cold air. Memory cue: a large gap means the wet bulb has been cooled strongly by evaporation, so the air is relatively dry.