By definition under RCRA hazardous wastes are ______ wastes.

Under the Resource Conservation and Recovery Act (RCRA), hazardous wastes are defined as solid wastes that can pose a significant risk to human health and the environment when improperly managed. This definition encompasses a wide range of materials that meet specific criteria, including those that are ignitable, corrosive, reactive, or toxic.

The term "solid waste" in this context does not exclusively refer to materials that are solid in form; it includes solids, liquids, and gases that are discarded or intended to be discarded, provided they meet the hazardous waste criteria. Therefore, identifying hazardous wastes as solid wastes aligns with the regulatory framework established by RCRA, which outlines how these materials must be handled and treated to ensure safety and compliance with environmental standards.

The other options do not apply to the RCRA definition. "Natural" does not encompass the variety of materials classified as hazardous under these regulations. "Gray" is not a recognized category within hazardous waste definitions, and "None of the above" fails to recognize that "solid" is indeed the term used in the context of RCRA. Thus, the correct understanding is that hazardous wastes, by definition under RCRA, are classified as solid wastes.