GRE Reading Comprehension

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Source: OG2

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, phytoplankton flourish only in surface waters where iron levels are sufficiently high. Martin 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 matter, but the carbon leaks back into the atmosphere when predators consume the phytoplankton and respire carbon dioxide.

Question List: 1

It can be inferred that the author of the passage mentions predators (line 10) primarily in order to

  • 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

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