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:

The United States made modifications and improvements to the Clean Air Act in 1990. The first steps to keep the air clean were taken earlier. The Environmental Protection Agency was created in 1970. Funding to study air pollution under the Clean Air Act was established earlier as well.

Answer:

Modifications and improvements were made to the Clean Air Act.