A law passed five years ago requires all new cars sold in

A law passed five years ago requires all new cars sold in Mauritania to be equipped with catalytic converters which decrease vehicular emissions of nitrogen dioxide. Emissions of nitrogen dioxide have significantly decreased throughout Mauritania, except in one particular city. Over the past 5 years, this particular city has had the highest rate of new car sales of any city in Mauritania, yet its nitrogen dioxide emissions remain unchanged from those prior to the passage of the above referenced law.

Which of the following best explains the discrepancy?

A. A higher percentage of cars made prior to the enactment of the law operate in the above city
B. The rate of new car sales has dropped moderately since the passage of the law
C. Nitrogen dioxide is not the only pollutant whose emissions have decreased since the passage of the law
D. The referenced law did not require owners of cars made before its passage to retrofit their cars with catalytic converters
E. The average drive time of the average car trip in this particular city does not allow sufficient drive time for the catalytic converters to reach optimal efficiency