Toxins are not produced by which of the following?

Toxins are substances that can cause harm if ingested or exposed to the human body, and they can come from various sources. The correct choice highlights that sand and crushed gravel do not produce toxins. These materials are inorganic and are typically used as construction aggregates or for filling purposes. They do not exhibit biological activity and do not release toxic substances.

In contrast, harmful microorganisms, such as certain bacteria, can produce toxins as metabolic by-products. This can often lead to foodborne illnesses. Chemical contamination involves substances like pesticides or heavy metals that can be harmful if ingested, but they are considered toxic based on chemical properties rather than biological production. Plants and seafood may also naturally contain toxins, such as certain alkaloids in plants or biotoxins in seafood, which can be harmful if consumed.

Thus, the characterization of sand and crushed gravel as not producing toxins is accurate and provides clarity in understanding the different sources and types of toxins encountered in food safety and environmental contexts.