Despite the Increasing Percentage of Newer Automobile Models on the Road

Multiple Choice

Despite the increasing percentage of newer automobile models on the road, air quality has not improved in some areas of the United States because:

A) even though NOx emissions from each vehicle have been reduced, there are many more automobiles on the road now.
B) even though there are fewer cars on the road today, more NOx is released due to increased burning of coal.
C) these gases are actually present in the atmosphere and cannot be reduced further.
D) we have no means to control the output of NOx.