The Clean Air Act legislation of 1970, 1977, and 1990 was designed to improve air quality in the United States by monitoring and reducing the emissions of air pollutants judged to pose threats to human health, such as carbon monoxide, nitrogen dioxide, sulfur dioxide, ozone, particulate matter, and lead. The main source of lead emissions in 1970 was the exhaust of vehicles burning gasoline to which tetra-ethyl lead had been added to improve combustion. By 1985, leaded gasoline was phased out of use, although airplanes and racecars were exempted. The 1990 amendments addressed the growing problem of urban smog by

The 1990 amendments addressed the growing problem of urban smog by acid rain.

Explanation:

As shown in the above question, the Clean Air Act was created with the goal of making the US air healthier, while also aiming to reduce the impacts of pollution on the American population. This law had as its main objective to reduce the emission of gases capable of polluting the atmosphere, especially those resulting from the burning of gasoline in cars. In 1990, a new amendment was added to this law. This amendment aimed to reduce emissions of sulfur dioxide and nitrogen oxides, as they were the main cause of acid pollution, which was responsible for acid rain that is extremely dangerous and negative for society.

1 a

2 d

3 a

4 c

explanation: