Question

nutrition food and wellness
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controlled - use substances
are limited to parts per thousand
are typically few in frequency and number
are on the generally recognized as safe (gras) list
have established guidelines for use in products

Explanation:

Controlled - use substances are those for which there are established rules and limits on their use in products. This is to ensure safety and proper functionality. The option "have established guidelines for use in products" aligns with the concept of controlled - use substances. The "parts per thousand" limit is too specific and not the general characteristic. "Few in frequency and number" does not define the nature of controlled - use substances. And being on the GRAS list means they are already considered safe without the need for additional "controlled - use" regulations.

Answer:

have established guidelines for use in products