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:


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.


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.


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.


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.


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.


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.