Since the 1960s, as a result of automobile modifications (as well as other factors),

Since the 1960s, as a result of automobile modifications (as well as other factors), carbon monoxide emissions in the U.S. have declined by more than three-fourths and emissions by more than 95 percent.

a. nitrogen and oxygenated
b. sulfur dioxide and bicarbonate
c. hydrocarbon and sulfur dioxide
d. nitrogen oxide and hydrocarbon
e. nitrogen hydroxide and oxygen