According to the following statement， can you tell me which term is described？（）
It is the air return for an air conditioned system. In most buildings， the area above a drop ceiling orunder a raised floor is used as the air return （source of air） for the air conditioning. Those drop
ceiling and raised floors are also where wire is often installed. If wire and cable is installed in a dropceiling or raised floor， it is out of sight. If that cable were burning， it would give off toxic fumes.These would be fed to the rest of the building by the air conditioner. As a result， people could beinjured even though they are a long way from the fire.