Solar ponds: OA tough to understand

[massi2884]

Can you explain why C is correct? Thanks.

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.

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.

Source: GMATPrep

[the_g_mat_ter]

Hi massy2884,

The conclusion from the last paragraph is that when they follow certain process (first evaporating water then diluting), they are able to destroy algae. To undermine this conclusion, we must chose a answer in such a way that it tells us that algae are destroyed due to some other reason and not because of this process.

A - Its talking about time lag but says algae are destroyed. The paragraph did not discuss time or did not say that algae are destroyed quickly. SO this choice would be out of scope for us.

B - This says that the lateral mobility was not impaired. SO what, algae was destroyed at last right.

C - The reason, other than what was cited in the paragraph. An alternate way. Algae was destroyed by microbes contained in the pond but not by the process followed. Hence Correct Answer.

D - This also cites the reason other than what was cited in the paragraph. But only difference is it talks about pressure difference that has been created by the evaporation and dilution process. Hence this process works and indirectly, this process can destroy algae.

E - Transparency was increased in the upper levels. So what. We dont have anything to do with this answer.

Hope its clear.