Which of the following substances must be managed as hazardous?

Used fuel filters must be managed as hazardous because they can retain residues of fuel, oil, and other potentially harmful substances that were filtered out. These residues may contain hazardous materials that can pose a risk to human health and the environment if not managed properly. The filters themselves may also be contaminated with toxic chemicals, making them a potential source of pollution.

Environmental regulations often categorize used fuel filters as hazardous waste due to the risk involved in their disposal and the need for specialized handling and storage. This ensures that they are treated, stored, and disposed of in accordance with safety standards to mitigate environmental impact and protect public health.

In contrast, fresh water is typically not considered hazardous; it is essential for life and generally poses no risk in its natural state. Stannous oxide may have certain safety considerations, but its classification would depend on the context of usage and exposure. Clean rags, when free from any hazardous substances, are usually regarded as safe and not classified as hazardous waste.