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题目材料：
Ground-level ozone, a by-product of gasoline combustion, stunts tree growth in laboratory conditions. Researchers tracked the growth rate of loblolly pine trees in a large forest in the southern United States from 1990 to 1994. Over the course of those five years, ground-level ozone in the forest increased, while the rate of circumferential trunk growth decreased. These results indicate that in nature as in the laboratory, no tree is likely to thrive in high concentrations of ground-level ozone.
Which of the following, if true, most seriously weakens the argument?
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AMeasurements of circumferential tree-trunk growth were taken of both saplings and mature trees.
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BConcentrations of ozone high up in the Earth's atmosphere fluctuated significantly less during the period from 1990 to 1994 than during the period from 1986 to 1990.
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CTraffic on a major interstate highway cutting through a large valley below the forest where the study was conducted increased sharply in 1990.
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DOf the many species of trees in the forest where the study was conducted, the loblolly pine was the only species to show a reduced rate of circumferential trunk growth from 1990 to 1994.
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ERainfall patterns in the area where the trees were growing did not change significantly over the five-year period from 1990 to 1994.
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