User Which class of fire-rated door would you use in this situation? An Opening in an exterior wall subject to moderate or light fire exposure from outside a building. Class AOpenings in firewalls and in walls dividing a single building into fire areas. Class BOpenings in enclosures of vertical communications through buildings and in 2-hour-rated partitions providing horizontal fire separations. Class COpenings in walls or partitions between rooms and corridors having a fire-resistance rating of one hour or less. Class DOpenings in exterior walls subject to severe fire exposure from outside a building. Class EOpenings in exterior walls subject to moderate or light fire exposure from outside a building