Classified hazardous locations include which of the following?

Hazardous locations are defined as areas where flammable gases, vapors, liquids, combustible dust, or ignitable fibers may be present, thereby creating a risk of fire or explosion. Classified hazardous locations are identified according to specific criteria relating to the types of materials present and the likelihood of these materials igniting.

In this case, all the environments listed—dry-cleaning plants, petroleum refineries, and sawdust-producing plants—can potentially contain hazardous materials. Dry-cleaning plants utilize solvents that can emit flammable vapors. Petroleum refineries process hydrocarbons which are inherently flammable and generate various hazardous conditions. Plants that produce sawdust encounter an additional risk due to the fine particles of wood dust that can create explosive atmospheres when suspended in air.

As a result, all mentioned locations are recognized under various classification codes, such as those outlined by the National Electrical Code (NEC) and the National Fire Protection Association (NFPA), reinforcing that each of these environments falls into the category of classified hazardous locations. This collective inclusion marks "all of these" as the correct choice, as they all meet the criteria for being in hazardous classifications due to their specific operations and the materials they handle.