GMAT GWD困惑的一道CR

GMAT GWD困惑的一道CR
Certain politicians in the country of Birangi argue that a 50% tax on new automobiles would halt the rapid increase of automobiles on Birangi's roads and thereby slow the deterioration of Birangi's air quality.Although most experts agree that such a tax would result in fewer Birangians buying new vehicles and gradually reduce the number of automobiles on Birangi's roads,they contend that it would have little impact on Birangi's air-quality problem.
Which of the floowing,if true in Birangi,would most strongly support the experts' contention about the effect of the proposed automobile tax on Birangi's air-quality problem.
d.Automobiles become less fuel efficient and therefore contribute more to air pollution as they age.
e.The scrapping of automobiles causes insignificant amounts of airi pollution.
我一开始选的D,很直观.但是后来我看到e,是啊,如果本来汽车产生的污染就不严重,那么即使这个计划有效,也不能解决多少这个城市的污染问题啊.那么为什么选D而不是e呢?