egmat wrote:
The correct answer - option D.
This is a paradox question.
- The ideal condition for algal bloom - river water with slow moving water
- When rainfall is lesser than normal - water level of rivers dip => rivers move more slowly
Thus, when rainfall is lesser than normal, amount of algae/unit of water increases (algae grows well).
Paradox: Despite the above, after a drought, even in very slow moving water, the algae levels are low
Pre-thinking:
Algae requires 2 parameters to grow well
1. River water habitat
2. Slow moving water
After a drought, we know that water movement is very slow. But what if the drought is so severe that, the river water habitat ceases to exist (completely dry)?
In such a case, the first parameter, river water habitat does not exist during the drought. This could explain why the algae level is low immediately after a drought (0 algae during the drought).
Lets look at the options.
(D) Australian rivers dry up completely for short intervals in periods of extreme drought.
In line with our thought process. This can explain the low algae level in the time following a drought
(A) During periods of extreme drought, the populations of some of the species that feed on algae tend to fall.
If this is true, then one would expect algae to thrive (algae's predator's population falls, hence the population of algae should increase). This goes in the opposite direction to our paradox.
We are trying to explain why algae population is low.
(B) The more slowly water moves, the more conducive its temperature is to the growth of algae.
This does not explain the paradox. By this logic, as the water speed tends to zero, the algae level should keep increasing.
C) There is no given on how living condition of other species can affect algae level.
This does not explain the paradox.
(E) Except during periods of extreme drought, algae levels tend to be higher in rivers in which the flow has been controlled by damming than in rivers that flow freely.
Basically, this tells us that algae tend to grow more in controlled flow rather than free flow. But this has no tangible impact on the contrast.
Hope this helps.
I chose A, since I thought it in the following way: if a certain species has as food source the algae, then if it couldn't find enough food, its population will decrease.
The first option states that the species that feed on algae decline which explains the low population of algae based on the reasoning above.
Could somebody add his two cents on it?
GMATCoachBen Bunuel avigutman KarishmaB manasp35