Nitrogen dioxide is a pollutant emitted by automobiles.

Nitrogen dioxide is a pollutant emitted by automobiles. Catalytic converters, devices
designed to reduce nitrogen dioxide emissions, have been required in all new cars in
Donia since 1993, and as a result, nitrogen dioxide emissions have been significantly
reduced throughout most of the country. Yet although the proportion of new cars in Donia’s capital city has always been comparatively high, nitrogen dioxide emissions
there have showed only an insignificant decline since 1993.

Which of the following, if true, most helps to explain the insignificant decline in nitrogen
dioxide emissions in Donia’s capital city?

A. More of the cars in Donia’s capital city were made before 1993 than after 1993.
B. The number of new cars sold per year in Donia has declined slightly since 1993.
C. Pollutants other than nitrogen dioxide that are emitted by automobiles have also
been significantly reduced in Donia since 1993.
D. Many Donians who own cars made before 1993 have had catalytic converters
installed in their cars.
E. Most car trips in Donia’s capital city are too short for the catalytic converter to
reach its effective working temperature.