A lot of people say that C is correct because: If the tour buses only spend about 1/4 of the time on driving from 1 site to another, then it means that about 3/4 of the time, the buses will idle next to the building. With a new parking lot, approximately 3/4 of the damage will go away because the buses will idle in the parking lot instead.
C强调的是idling的时间有四分之三——强调了闲逛的时间很长,如果减少这个闲逛时间,会大大减少exhaust
E说的是一些旅游巴士在找不到停车位的时候会去闲逛——没有说这个闲逛的时间是多久,而且E选项在原文中已经有陈述了:'so most buses have idled at the curb during each stop on their tour'