Question

the standards for pollutants are based on ___.
congresss opinion
the presidents opinion
scientific studies
public opinion

Explanation:

Pollutant standards are established through scientific research. Congress and the President may have roles in policy - making, but the core basis is scientific evidence. Public opinion can influence but is not the fundamental basis. Scientific studies provide data on pollutant impacts, toxicity, etc., which are essential for setting standards.

Answer:

C. scientific studies