Why Have Efforts to Reduce Sulfur Oxide Emissions Met with Greater

Why have efforts to reduce sulfur oxide emissions met with greater success than those directed at nitrogen oxide emissions reductions?

A) Sulfur oxides have higher molar masses than nitrogen oxides.
B) Sulfur oxide emissions come from a limited number of point sources.
C) Nitrogen oxides are only produced at power plants.
D) Nitrogen reacts more readily with oxygen than sulfur.