[求助] Prep 1- 17&18

大家帮我看看这两道题啊~~~~实在是想不出~~~~帮帮我!~~~~谢谢~~~

17. (27669-!-item-!-188;#058&003302)

Urban air contains more sulfur dioxide than does rural air, and plants in cities typically grow more slowly than do plants in rural areas.  In an experiment to see how much of the difference in growth is due to sulfur dioxide, classes in an urban and a rural school grew plants in greenhouses at their schools and filtered the greenhouse air to eliminate sulfur dioxide.  Plants in the urban greenhouse grew more slowly than those in the rural greenhouse.

Which of the following, if true, would it be most important to take into account in evaluating the result?

(A) The urban school was located in a part of the city in which levels of sulfur dioxide in the air were usually far lower than is typical for urban areas.
(B) At both schools, the plants in the greenhouses grew much more quickly than did plants planted outdoors in plots near the greenhouses.
(C) The urban class conducting the experiment was larger than the rural class conducting the experiment.
(D) Heavy vehicular traffic such as is found in cities constantly deposits grime on greenhouse windows, reducing the amount of light that reaches the plants inside.
(E) Because of the higher levels of sulfur dioxide in the air at the urban school, the air filters for the urban school's greenhouse were changed more frequently than were those at the rural school.

为什么选D呢~~~~D的such as后面是不是少了一个单词啊...我怎么看不明白~~~~~

18. (27717-!-item-!-188;#058&003330)

Many small roads do not have painted markings along their edges.  Clear edge markings would make it easier for drivers to see upcoming curves and to judge the car's position on the road, particularly when visibility is poor, and would therefore seem to be a useful contribution to road safety.  However, after Greatwater County painted edge markings on all its narrow, winding roads, the annual accident rate along those roads actually increased slightly.

Which of the following, if true, most helps to explain the increase in accident rate?

(A) Greatwater County has an unusually high proportion of narrow, winding roads.
(B) In bad weather it can be nearly as difficult for drivers to see the road as it is at night.
(C) Prior to the painting of the edge markings, Greatwater County's narrow, winding roads already had a somewhat higher accident rate than other Greatwater County roads.
(D) Many of the accidents on narrow, winding roads involve a single vehicle veering off the road, rather than the collision of two vehicles.
(E) After the markings were painted on the roads, many drivers who had gone out of their way to avoid driving on those roads at night no longer did so.

也不明白为什么这题选E!!~~

多谢各位回帖的朋友~~~

17题选D的原因是，这道题问的是对结论起到重要作用的一个考虑因素是什么？也就是这个因素会对实验的准确性起到影响。那D说的是城市里交通频繁，造成对greenhouse的玻璃的污染，遮住了光。那植物是靠光合作用的嘛，遮住了光肯定要对实验结果有影响。其他选项都不会对实验结果造成影响。

18题是问下列那个选项对事故率上升做了解释？那安装题中意思，对路沿做标记应该是起到加强安全的作用，那为什么greatwater的事故率上升了呢。那就要找到其中的一个矛盾：即E说的路沿做了标记之后，一些以前觉得不安全选择其他道路的司机不在这么做了，那就是他们又选回了这条小路。以前都没人敢走的一条路，现在有开始有很多人走了，事故率有slightly的上升是合理的啊。

mycat2 mm真是太感谢你了！！

你分析的好细啊~~~我明白了~~~谢谢谢谢！