请教prep 07的第17题

孔方笑

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.  lants 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.

关于D，既然实验的目的是 to see how much of the difference in growth is due to sulfur dioxide，那么引入他因，只能说明，影响生长速度的不只是sulfur dioxide，还有amount of light，不能说明how much of the difference in growth is due to sulfur dioxide。 打个比方，在一个测试牛奶对小孩身高影响的试验中，实验者把小孩分成两组，在不让小孩喝牛奶的前提下，一组经常打篮球一组不打，最后发现打篮球的小孩长得比较高，这个，完全不能说明牛奶的作用啊。难道其实这题的逻辑是在最终evaluating的时候，需要排除他因，所以发现他因对最后的结果有帮助？这也太绕了吧？