PyjamaScientist
AndrewN,
Could you share your views on Question 4? I was split between (C) and (D), and neither made much sense to me. I chose (D) eventually. Seems like GMAC took a good leap of assumption in qualifying the option (C) as the correct choice here.
What are your thoughts?
Hello,
PyjamaScientist. I imagine this passage comes from one of the paid exams. In any case, I have never seen this question set before. I am not sure if anyone else has written on question 4 above. If so, I mean no disrespect. I just want to write my own analysis and explain what led me to select the correct answer in 1:12. I approached the question more like it came from CR, and since it referred to the
last paragraph only, I made that my passage.
Quote:
4. Which of the following, if true, would seriously undermine the validity of the conclusions drawn from the experiment described in the last paragraph of the passage?
If we want to
undermine the validity of the conclusions drawn from the experiment, it is vital that we understand just what conclusions were drawn. The last paragraph of the passage:
Quote:
A recent experiment has supplied a more promising method for controlling the algae. To repress the algae cells' capacity for accommodating themselves to environmental changes, the water in the solar pond was first made more saline through evaporation and then diluted by a rapid inflow of fresh water. This shock reduced the cells' ability to regulate the movement of water through their membranes. They rapidly absorbed water, resulting in distortions of shape, increase in volume, and impairment to motility. Their buoyancy adversely affected, the cells sank to the bottom of the pond, where they encountered the hot waters of the storage layer and were destroyed. This method allows for effective control of nuisance algae while leaving solar ponds as one of the cleanest technologies providing energy for human use.
The conclusions can be found, rather conveniently, in the final line. The method used in the experiment
allows for effective control of nuisance algae. This is the first conclusion. Then, we get a second conclusion. The method also
[leaves] solar ponds as one of the cleanest technologies providing energy for human use. The method itself is the only common element to the two conclusions. But if
nuisance algae muddy up the waters, so to speak, then there is a relationship between getting rid of these algae and cleaning up the solar ponds. If we understand these conclusions, then the answer choices ought to be easier to navigate.
Quote:
(A) The algae cells that sank to the bottom of the pond were destroyed
only after a time lag of twenty-four hours.
The important part is that
the algae cells... were destroyed, not the
time lag. The conclusions are not based on how quickly the algae were destroyed, so we cannot make heads or tails of this twenty-four-hour turnaround time. This should prove an easy elimination.
Quote:
(B) The
lateral motility of the algae cells that sank to the bottom of the pond
was not impaired.
Nothing goes against the passage here, which tells us that the algae suffered
impairment to motility:
impairment means diminishment, not a complete breakdown. Furthermore, the sentence following the one that mentions motility informs us that the algae cells'
buoyancy [was] adversely affected, so they sank and
were destroyed. Thus, the conclusions are based on
vertical motility, if we want to think in such terms. What the algae cells do from side to side makes no difference to the conclusions. If the algae sink, they are destroyed, even if they writhe in agony from side to side on the pond floor. We need to keep looking for a weakener.
Quote:
(C) The water with which the artificial solar pond was diluted
contained microorganisms that kill algae.
Stop the press: if some unacknowledged factor
[killed the] algae, then the conclusions based on the experiment may be premature. In other words, the intermediate conclusion that
this shock reduced the cells' ability to regulate the movement of water through their membranes may not be accurate, because the saline-to-freshwater environmental change,
in and of itself, would not explain the experimental results. Simply stated, the shock-to-the-system conclusion could be completely inaccurate if
microorganisms that kill algae came in with the freshwater and went to work. Regarding the method used in the experiment, then, perhaps increasing the salinity of the solar ponds was a waste of time and resources. Only the addition of freshwater laden with the microorganisms might be necessary to kill the algae.
Quote:
(D) The algae cells that sank to the bottom of the pond were
actually killed by the rapid change in pressure.
This seems similar to the previous answer choice—something else killed the algae—so what makes this option incorrect? Look at the line in question:
Their buoyancy adversely affected, the cells sank to the bottom of the pond, where they encountered the hot waters of the storage layer and were destroyed. Now, my interpretation could admittedly be limited in scope, since I read just the last paragraph. (I wanted to save the passage and other questions for when I might encounter them in my own practice tests.) However, I do not see the line from the passage as necessarily implicating
the temperature of the
hot waters of the storage layer in killing the algae; I see the line as two separately reported pieces of information: 1) the algae sink to a particular level known as
the storage layer, where the water is hot; 2) the algae
were destroyed. Based on the earlier information in the paragraph, it seems to me as though the algae were already on their way out, sinking as they were. What
exactly may finish them off within the hot waters of the storage layer does not affect the conclusions. The algae still sink to that layer and are destroyed. That is the major difference to my eye between this answer choice and (C) above.
Quote:
(E) The higher salinity brought about through evaporation
increased the transparency of the upper levels of water in the pond.
If this information ties into something mentioned earlier in the passage, it was lost on me, since, again, I read only the final paragraph. In any case, the
transparency of the water has no bearing on the experiment and thus does not concern us.
Perhaps the question makes more sense now, at least through the lens of confining all information about the passage to the last paragraph. Thank you for thinking to ask.
- Andrew
_________________
I am no longer contributing to GMAT Club. Please request an active Expert or a peer review if you have questions.
5%
(low)
17%
(03:04) wrong 
95%
(hard)
Updated on: 18 Sep 2017, 09:32 
