The correct answer is motor vehicles.
The Clean Air Act primarily intended to set the parameters or standards for ambient air quality in the United States and plans associated with implementation of these standards. These are to prevent pollution caused by motor vehicles.
It requires air pollution controls to be installed on motor vehicles' that would assist in maintaining the air quality standards and keep the air safe for the citizens.
Motor vehicles, chimneys, and drainage pipes are all examples of point source pollution.
Therefore, the major pollutants are released by motor vehicles causing the highest level of pollution.
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