Explanation

5. All three choices are correct. Martin’s hypothesis was that adding iron to iron-poor regions of the ocean could help alleviate global warming.
Choice A is correct: the passage presents Martin as using the standard understanding of how phytoplankton photosynthesize as a basis for the hypothesis.
Choice B is correct: the passage states that experiments confirmed that adding iron to iron-poor regions increases phytoplankton growth in those regions. Therefore,
Martin’s prediction about this was correct.
Choice C is correct: it can be inferred that in Martin’s hypothesis the means by which adding iron in certain regions could alleviate global warming is that phytoplankton
increase in those regions and absorb atmospheric carbon dioxide. The passage states that predators who consume phytoplankton respire carbon dioxide, so that
the carbon dioxide absorbed by phytoplankton reenters the atmosphere. Therefore, Martin’s prediction about this was incorrect.

6. 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

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.