Solar ponds are bodies of water in which circulation is incomplete and

Question

Solar ponds are bodies of water in which circulation is incomplete and there is a very high salt concentration that increases with depth. This vertical change in salinity serves to trap heat because concentrated brine in the lowest water level acts as a collector and storage area for solar heat, while the less saline, lighter water at the upper levels provides insulation. Heat is thus retained in the depths.

An artificial pond of this type has been constructed on the western shore of the Dead Sea in Israel in order to test its suitability as a source of low-grade heat for conversion into electricity. An immediate threat to the success of the venture was the growth of algae. Water in solar ponds must be kept maximally transparent to allow penetration of light to the deep storage area. Therefore, any particles of matter in the water, such as algae cells, that scatter or absorb light will interfere with the collection of heat.

One proposed method of controlling the algae was the application of an algicide. However, the Dead Sea is a closed body of water without any outlet and as such is very easily contaminated. Extensive use of chemicals in numerous future full-scale solar ponds would lead to such contamination of the Dead Sea, which now enjoys a lucrative tourist trade.

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.   1 
 1.The primary purpose of the passage is to

(A) discuss ways of solving a problem that threatens to limit the usefulness of an energy source
(B) explain the mechanisms by which solar heat may be converted into energy
(C) detail the processes by which algae cells colonize highly saline bodies of water
(D) report the results of an experiment designed to clean contaminated bodies of water
(E) describe the unique properties of a solar pond on the edge of the Dead Sea

2 
 2. It can be inferred from the passage that which of the following is true about the salinity and temperatures of the highest and lowest water layers in a typical solar pond?

(A) The bottom layer is both highly saline and quite hot, while the top layer is less saline and cooler.
(B) The two layers have similar salinity levels, but the bottom layer is hotter than the top.
(C) There is no way to predict the salinity and temperature of the different water layers in different solar ponds.
(D) The bottom layer is less saline and quite hot, while the top layer is more saline and cooler.
(E) The top layer has both higher salinity and higher temperatures than the bottom layer.

3 
 3. According to the passage, the growth of algae was considered a threat to the success of the artificial pond near the Dead Sea because the algae

(A) produce excess oxygen that lowers the water temperature in the pond
(B) restrict the circulation of water within the pond
(C) enable heat to escape through the upper level of the pond
(D) prevent light from penetrating to the lowest levels of the pond
(E) prevent accurate measurement of the heat collected in the pond

4 
 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?

(A) The algae cells that sank to the bottom of the pond were destroyed only after a time lag of twenty-four hours.
(B) The lateral motility of the algae cells that sank to the bottom of the pond was not impaired.
(C) The water with which the artificial solar pond was diluted contained microorganisms that kill algae.
(D) The algae cells that sank to the bottom of the pond were actually killed by the rapid change in pressure.
(E) The higher salinity brought about through evaporation increased the transparency of the upper levels of water in the pond.

JarvisR · Accepted Answer

Time taken 7 mins

1.The primary purpose of the passage is to
A. discuss ways of solving a problem that threatens to limit the usefulness of an energy source

2. It can be inferred from the passage that which of the following is true about the salinity and temperatures of the highest and lowest water layers in a typical solar pond?
A. The bottom layer is both highly saline and quite hot, while the top layer is less saline and cooler.

3. According to the passage, the growth of algae was considered a threat to the success of the artificial pond near the Dead Sea because the algae
D. prevent light from penetrating to the lowest levels of the pond

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?
C. The water with which the artificial solar pond was diluted contained microorganisms that kill algae.
D. The algae cells that sank to the bottom of the pond were actually killed by the rapid change in pressure.
E. The higher salinity brought about through evaporation increased the transparency of the upper levels of water in the pond.

Conclusion:
  This method allows for effective control of nuisance algae while leaving solar ponds as one of the cleanest technologies providing energy for human use.
 
Argument can be weaken in following way by proving that:
1) the experiment did pollute the water.
2) The experiment is not as effective as it is claimed.
etc.
 
C,E seems to be the only relevant options.

Option C:
 This means that new factor introduced in C can pollute/contaminate the DS.

Option E: The higher salinity brought about through evaporation increased the transparency of the upper levels of water in the pond.
 >>Strengthen.

pra1785 · Answer

For Question 4, can someone explain why option D is wrong? 
The author states that the change in salinity eventually killed the algae. Option D says that the rapid change in pressure did.... Isn't this option weakening the validity of the conclusion ?

arvind910619 · Answer

Hi I can help

Here is the para in which experiment is described

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.

Here the conclusion is that the introduction of saline water and evaporation resulted in the increase in salinity which in turn affected algae .
But if the distilled water contained micro organisms that killed algae then the conclusion is weakened as the reason for the death of algae is other than what the author had cited

KaranB1 · Answer

Below is the explanation for question no 4.

The conclusion of the author in the 4th Paragraph is "A recent experiment has supplied a more promising
method for controlling the algae.".

Option C is implying that algae was controlled due to an alternate reason hence weakening the conclusion of the author.

krittapat · Answer

GMATNinja   KarishmaB 
Could you please help explain question 4?
I answered B because I thought that if the algae cells were not impaired when they sank to the bottom of the pond, such method would not be effective in controlling nuisance algae.

ReedArnoldMPREP · Answer

Answer B says the 'lateral motility' of the cells that sank was not impaired, not that the cells weren't damaged at the bottom. 'Lateral motility' is basically 'side to side movement,' which is not relevant to the discussion here.

Answer C is right, though I admit I'm not fully on board with the 'why.' The best I can do is that the presence of microorganisms might contaminate the water in the detrimental way discussed in the previous paragraph.

PyjamaScientist · Answer

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?

AndrewN · Answer

QUESTION FOUR

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.

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    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   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    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.

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

GMATGuruNY · Answer

This is a PLANNING argument.
The question at hand is as follows:
If we follow the given method, will algae be controlled?

B: The lateral motility of the algae cells that sank to the bottom of the pond was not impaired. 
Here, the answer is YES, since any horizontal movement by the algae cells will not protect them from  the hot waters of the storage layer  when they are forced to the bottom of the pond.

D: The algae cells that sank to the bottom of the pond were actually killed by the rapid change in pressure. 
Here, the answer is also YES, since  the rapid change in pressure  will kill the algae cells when they are forced to the bottom of the pond.

C: The water with which the artificial solar pond was diluted contained microorganisms that kill algae. 
Only here is the answer NO, since the given method does NOT suggest the use of water with these killer  microorganisms .

C

SwethaReddyL · Answer

@GMATNinga for the 4th question i couldn't understand why B is wrong. I eliminated C, because C says that algae is getting killed, this doesn't weaken the conclusion of the research right? although i didn't fully understand B, i picked via poe
Please tell me how do i avoid such mistakes 
Awaiting your reply

guddo · Answer

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?

The passage says the experiment supports a clean, effective way to control algae: change the salinity, then rapidly dilute the pond, causing the algae to lose control of water movement, sink, reach the hot lower layer, and die.  To weaken that conclusion, we need evidence that the algae were not actually destroyed by this proposed mechanism.  The best way is to show that  something else  killed them.

(A) The algae cells that sank to the bottom of the pond were destroyed only after a time lag of twenty-four hours.

This does not seriously weaken the conclusion. Even if destruction took some time, the method could still be the reason they died.

(B) The lateral motility of the algae cells that sank to the bottom of the pond was not impaired.

This is too narrow. The passage only needs the cells to lose enough effective control and buoyancy to sink. Lack of impairment in lateral movement does not directly undermine that.

(C) The water with which the artificial solar pond was diluted contained microorganisms that kill algae.

This is the best answer. If the fresh water already contained organisms that kill algae, then the experiment does not show that the salinity-shock method itself caused the algae’s destruction. That would seriously undermine the validity of the conclusion.

(D) The algae cells that sank to the bottom of the pond were actually killed by the rapid change in pressure.

This does not hurt the conclusion much. The method would still have killed the algae, even if the exact mechanism was pressure rather than heat.

(E) The higher salinity brought about through evaporation increased the transparency of the upper levels of water in the pond.

This does not undermine the conclusion. If anything, it may help the pond function better.

Answer: (C)

Question 4 is also explained in the following posts:

# 1 
 # 2 
 # 3 
 # 4 
 # 5

Solar ponds are bodies of water in which circulation is incomplete and

Prep Toolkit

Top 5 GMAT Debrief Videos

615 to 715 in 15 Days (Ria's Strategies)

More than Quant/Verbal Abilities: Speed vs. Accuracy

745 with Only Self-Prep and GMAT Club

From 555 to 765: 210-point Improvement

Perfect 805 Score Debrief by Julia

My Rewards

GMAT Reading Comprehension (RC) Questions

Solar ponds are bodies of water in which circulation is incomplete and

Prep Toolkit

615 to 715 in 15 Days (Ria's Strategies)

More than Quant/Verbal Abilities: Speed vs. Accuracy

745 with Only Self-Prep and GMAT Club

From 555 to 765: 210-point Improvement

Perfect 805 Score Debrief by Julia

My Rewards