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.
我觉得此题没有对的，官方给的答案是E,说的是那个c东西能不能有效的工作，但我认为原文没有说他的有效性啊，怎么解释呢？

1.原文“Yet although the proportion of new cars in Donia’s capital city has always been comparatively high,”可排除A,B。

2.C与问题无关，1993年以来汽车尾气其它污染物显著降低与二氧化氮排放多少没有关联。

3.D与原文“Catalytic converters, devices designed to reduce nitrogen dioxide emissions, have been required in all new cars in Donia since 1993”明显矛盾。

4.E，大部分汽车在 Donia’s capital city 为短程，catalytic converter 达不到有效工作需要的温度，也就不能减少污染。 原文是没提及此事，但请注意该题的问题含Which of the following, IF TRUE...

拙见，望指正