Congress passed the Clean Air Act in 1970. Since this act was passed, emissions of the six main air pollutants Group of answer choices cannot be measured since Congress failed to appropriate money to monitor the level of emissions. have fallen by more than one-half. have remained essentially constant, even though significant economic growth has occurred in the United States since 1970. have increased significantly due to the growth of the U. S. economy.

have fallen by more than one-half.

Explanation:

Thick, noticeable smog in many of the cities and manufacturing centers of the United States helped bring the 1970 e Clean Air Act to spur passage at the height of the national environmental movement. The ensuing modifications were intended to enhance its effectiveness and address newly recognized problems of air pollution such as acid rain and stratospheric ozone layer damage. The Clean Air Act allows EPA to set national environmental air quality levels for certain specific and prevalent contaminants, based on the latest research, to protect public health and welfare nationally. Certain main regulations are intended to reduce the rise in emissions from increasing motor vehicle numbers and new manufacturing plants.