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  3. When marine organisms called phytoplankton photosynthesize, they absorb carbon dioxide dissolved in seawater, potentially causin

When marine organisms called phytoplankton photosynthesize, they absorb carbon dioxide dissolved in seawater, potentially causin

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问题 When marine organisms called phytoplankton photosynthesize, they absorb carbon dioxide dissolved in seawater, potentially causing a reduction in the concentration of atmospheric carbon dioxide, a gas that contributes to global warming. However, phy-
line toplankton flourish only in surface waters where iron levels are sufficiently high. Mar-
5 tin therefore hypothesized that adding iron to iron-poor regions of the ocean could help alleviate global warming. While experiments subsequently confirmed that such a procedure increases phytoplankton growth, field tests have shown that such growth does not significantly lower atmospheric carbon dioxide. When phytoplankton utilize carbon dioxide for photosynthesis, the carbon becomes a building block for organic
10 matter, but the carbon leaks back into the atmosphere when predators consume the phytoplankton and respire carbon dioxide.
It can be inferred that the author of the passage mentions predators(line 10)primarily in order to
选项 A、help explain why Martin’s hypothesis is incorrect
B、identify one effect of adding iron to iron-poor waters
C、indicate how some carbon dioxide is converted to solid organic matter
D、help account for differences in the density of phytoplankton between different regions of the ocean
E、point out a factor that was not anticipated by the scientists who conducted the field tests mentioned in the passage
答案A
解析 Lines 7-11 of the paragraph present the evidence against Martin’s hypothesis. Lines 7-8 present field test results showing that Martin’s hypothesis is incorrect, and the last sentence explains these results: the reason the increased phytoplankton resulting from the addition of iron do not reduce atmospheric carbon dioxide is that while the phytoplankton absorb carbon dioxide, the gas reenters the atmosphere when it is respired by phytoplankton predators. Therefore Choice A is correct: predators are mentioned to explain why Martin’s hypothesis is incorrect. Choice B is not correct because while predators’ consumption of phytoplankton and respiration of carbon dioxide might be considered one indirect consequence of adding iron to iron-poor waters, identifying a consequence is not the primary function of the mention of predators. Choice C is incorrect because the reference to predators is used to explain how carbon dioxide reappears as a gas, and Choice D is incorrect because no connection is suggested between predators and the distribution of phytoplankton. Choice E is not correct because it is Martin who did not anticipate this factor, rather than the scientists who conducted the field tests.
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