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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.
In an experiment to see how much of the difference in growth is due to sulfur dioxide 说明这个实验是为了验证二氧化硫对生长速率有什么影响。
第一次选了B，我想了想，可能拿温室的植物和室外的植物对比，并不能说明什么结果吧？毕竟根据控制变量实验的定义，温室里除了二氧化硫被排掉了，其他因素包括温度、光照可能都跟室外的不同（题目中并未提到），所以根据这个对比不能得出二氧化硫对植物生长速率有何影响的结论吧。
相比起来，D更像是像控制变量实验定义而靠拢，根据可能存在的某个定量不同，而导致实验结果发生偏差，这对实验结果是有意义的。
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