单项选择题 Freon 12 which is used to produce intense cold is normally a （） when at atmospheric pressure and temperature.

单项选择题 The quantity of cooled air delivered by an air conditioning unit should （） the sum of the quantity of air re-circulated to the unit and the quantity mechanically exhausted.

单项选择题 The shutter of the evaporator was blocked with （）, it means there was too much evaporation.