Two decades of clean air laws have knocked down levels of one contaminant after another: Sulfur dioxide and carbon monoxide emissions are down 37 percent;
(Sulfur dioxide and carbon monoxide emissions; are; down 37 percent) Context(Sulfur dioxide and carbon monoxide emissions are,List([89, 137))):(Two decades of clean air laws; have knocked down; levels of one contaminant after another)
Sulfur dioxide and carbon monoxide emissions are,List([89, 137))
Sulfur dioxide and carbon monoxide emissions Two decades of clean air laws
are have knocked down
down 37 percent
levels of one contaminant after another