Hazardous exhaust systems are regulated under the IMC provisions for special exhaust because they handle potentially dangerous fumes, vapors, dusts, or other contaminants. The code requires the duct system and its support components to be suitable for that hazard level, and supports for hazardous exhaust duct must be constructed of noncombustible material . That is why Option C is correct. Plastic and fibrous supports would not satisfy the required fire and safety performance, and UL 181 applies to certain factory-made air ducts and connectors, not as the governing support-material rule for hazardous exhaust duct supports.