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题库 / Prep2007E1-CR-17
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?
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.
At both schools, the plants in the greenhouses grew much more quickly than did plants planted outdoors in plots near the greenhouses.
The urban class conducting the experiment was larger than the rural class conducting the experiment.
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.
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.
B选项即使正确，这个差值（由于urban和rural本质导致的）的无关变量也可以消除，两次差值做减法即可得出二氧化硫的影响
A选项同理，这个实验已经滤掉了二氧化硫，得出的一切差值是无关变量的影响，urban school二氧化硫浓度的高低无所谓，反正都会过滤掉
CE选项无关
D选项引入了一个除二氧化硫之外变量，故试验结果不准确了
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