Which of the following amendments significantly expanded what was addressed under the Clean Air Act?

The amendment in 1990 is significant because it introduced comprehensive changes that greatly expanded the scope of the Clean Air Act. This amendment established new regulatory programs aimed at addressing a range of air quality issues, including the regulation of acid rain, ground-level ozone, and toxic air pollutants. The 1990 amendments also created provisions for the establishment of a market-based approach to reduce emissions of sulfur dioxide, allowing for a cap-and-trade system. This marked a pivotal shift in how air quality was managed, emphasizing both public health and environmental protection.

In contrast, earlier amendments, such as those from 1970 and 1963, were foundational in establishing the initial frameworks for air quality standards and federal oversight. While they laid the groundwork for air pollution control, they did not implement the same expansive regulatory measures or the comprehensive structure that the 1990 amendment did. The 1980 amendments also did not have the same level of impact as those introduced in 1990, focusing more on certain aspects of control rather than a broad overhaul. Thus, the 1990 amendment stands out for its sweeping reforms and enhancements to air quality regulations.