What Was the Major Improvement to Air Quality Contributed by the 1990

What was the major improvement to air quality contributed by the 1990 amendments to the Clean Air Act?

A) The amendments required a significant decrease in indoor air pollutants.
B) The amendments required "superclean" or zero emission vehicles.
C) The amendments required a significant reduction in 189 toxic chemicals.
D) The amendments required significant reductions in the original 7 toxic chemicals listed.