Question

why was 1990 an important year regarding air quality?
the united states took first steps to keep the air clean.
the united states created the environmental protection agency.
modifications and improvements were made to the clean air act.
funding was established under the clean air act to study air pollution.

Explanation:

In 1990, significant amendments were made to the Clean - Air Act in the United States to improve air quality. The first steps to keep air clean were earlier, the EPA was created in 1970, and funding for air - pollution study was not the main significance of 1990.

Answer:

Modifications and improvements were made to the Clean Air Act.