Provide your best answer for the following question. Give ONLY the answer, no other words or explanation.
For example:

Answer: <most likely answer, as short as possible; not a complete sentence, just the answer!>.

The question is: The Water Quality Act of 1987 required the development of standards that would prevent toxic pollutants from reaching dangerous levels in bodies of water. This act affected the amounts of toxic chemicals that could be discharged into bodies of water. Which of the following would have been most regulated by this act?
paper mills
Toxic chemicals.