[求助]PREP 1-17

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.

答案是D 為什麼不是E呀?

我觉得是B~

谁帮帮吧

D 是说要排除不是因为车排出的废气遮盖了阳光而引起的成长速度的不同。

似懂非懂耶~~~  我的閱讀能力不太行~~~

有沒有人想再討論一下~~

原文说城区空气比郊区空气含有更多的sd，所以城区的植物比郊区的植物长得慢。为了检测sd对植物生长起多大作用，在城区和郊区的学校在都温室里种植植物，并将sd排除出温室。实验结果表明在城区温室中生长的植物比在郊区温室中生长的慢。

题目问评估这个结果时最应该考虑下列哪个因素。

注意实验的目的是要检测sd对植物生长的作用，所以实验中除了sd之外对植物生长起到作用的因素都应该考虑进来，否则其他因素和sb混淆在一起就不能得到准确的结论。

D说城市中由于交通工具多造成温室的窗户上积累了污垢，使得植物生长所需要的光的数量减少。如果这样的话我们就不能把在城市温室中植物生长的速度完全归结为sd的作用。所以这个因素应该得到考虑，为正确答案。