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i think its D. the oceanographers conclusion is that plankton concentrations are low because of lack of iron. the question basically asks us to find the reason against concluding why this may not be the case .... D gives a reason why the oceanographers theory may be true, but wasnt demonstrated

A for me,

D does strengthen the hypothis!

IMO - D.

D IMO.

IMO D.

IMO D . The only options which does not weakens the oceanographers hypothesis ..others do .

E "The iron compounds released into the area occur naturally in areas of the ocean
where plankton thrive." says that the iron rich compounds needed for the planktons occur naturally ...but does it mean that if given artificially they will not help planktons to survive ?

A "Not all of the nitrogen compounds that are sometimes found in relatively high
concentrations in the oceans are nutrients for plankton."

Although a good one ..got confused between A & D , but in the end choose D since A says that not all of the compunds found in high concentration are nutrients for plankton , but it does not says that the compounds which are necessary are not found in those areas ....

Second thought, D. misguided by question!

A for me... whats the OA?

Q19:
Plankton generally thrive in areas of the ocean with sufficient concentrations of certain
nitrogen compounds near the surface, where plankton live. Nevertheless, some areas,
though rich in these nitrogen compounds, have few plankton. These areas have
particularly low concentrations of iron, and oceanographers hypothesize that this shortage
of iron prevents plankton from thriving. However, an experimental release of iron
compounds into one such area failed to produce a thriving plankton population, even
though local iron concentrations increased immediately.

Which of the following, if true, argues most strongly against concluding, on the basis of
the information above, that the oceanographers’ hypothesis is false?

A. Not all of the nitrogen compounds that are sometimes found in relatively high
concentrations in the oceans are nutrients for plankton.
B. Certain areas of the ocean support an abundance of plankton despite having
particularly low concentrations of iron.
C. The release of the iron compounds did not increase the supply of nitrogen
compounds in the area.
D. A few days after the iron compounds were released, ocean currents displaced the
iron-rich water from the surface.
E. The iron compounds released into the area occur naturally in areas of the ocean
where plankton thrive.

****
1. The question specifically asks about a statement that counters the implicit conclusion from the passage - "The oceanographers’ hypothesis is false because even the introduction of iron did not induce plankton population surge".

2. This means the possible answer should include a phrase that states that Iron does help the plankton population growth.

3. Now let us review the answers one by one:

A. Not all of the nitrogen compounds that are sometimes found in relatively high
concentrations in the oceans are nutrients for plankton.
*This helps our analysis because it questions altogether the nitogen source it self but doesnot say anything about Iron involvement - hence can be eliminated.
*
B. Certain areas of the ocean support an abundance of plankton despite having
particularly low concentrations of iron.
*This can be eliminated directly as it doesnot say anything that supports our claim.*

C. The release of the iron compounds did not increase the supply of nitrogen
compounds in the area.
*This actually supports the implicit conclusion of the author hence can be eliminated"

D. A few days after the iron compounds were released, ocean currents displaced the
iron-rich water from the surface.
*This counters the conclusion that iron is not present for thriving plankton population" - Best choice till now.

E. The iron compounds released into the area occur naturally in areas of the ocean
where plankton thrive.
*This doesnot provide any additional information other than the information inferred from the Oceanographer's hypothesis. - can be eliminated.*

So, it can be safely concluded that the best answer is D.

I was between D and E.

in D, I didnt like the 'a few days after' note - what if planktons thrive immediately after iron compounds are released, even before the currents can displace them? (i know, thats a stetech...(
in E, i feel the subtle use of 'naturally' implies something. maybe the researchers are not using the 'right' iron compound for planktons to thrive?

I chose E. whats the OA?