Congress Passed the Clean Air Act in 1970

Congress passed the Clean Air Act in 1970.Since this act was passed, emissions of the six main air pollutants

A) have fallen by more than one-half.
B) have increased significantly due to the growth of the U.S. economy.
C) cannot be measured since Congress failed to appropriate money to monitor the level of emissions.
D) have remained essentially constant, even though significant economic growth has occurred in the United States since 1970.