When there is less rainfall than normal, the water level of Australian

(A) During periods of extreme drought, the populations of some of the species that feed on algae tend to fall.
if the populations of species that feed on algae tend to fall ---> less feeding on algae --> algae population will increase.

We want to resolve the paradox ( we want to find a reason why algae population goes down during extreme drought).
D is a clear winner in this case.

Premises:
(1) Habitat of algae is river water.
(2) Algae grow best in slow-moving water, when there is less rainfall than normal.
(3) After extreme drought, even in very slow-moving river water, algae levels are low.

Question is to resolve the apparent paradox on low level of algae in slow-moving water when it is after extreme drought.

A) If number of algae-eaters become less, algae level is supposed to be higher. Does not give any information on why algae level becomes lower following drought.
B) This says how algae level can be higher. There is no information on why algae level becomes lower following drought.
C) There is no given on how living condition of other species can affect algae level.
D) The statement implies that dry riverbed, which does not provide normal living condition for algae, triggers death of algae to some extent. Immediately after death of some algae, even though there is very slow-moving river water following recovery from drought, it is natural that algae level would be low. The given information here resolves the paradox.
E) There is no information on the factors affecting given situation, i.e., algae level, following a period of extreme drought, in slow-moving river water.

Correct answer is D.

Less Rainfall ---- > Water Level of Australia falls -------> Rivers flow slowly -------- > Increase in amount of algae per unit area.

Extreme drought ------- > Level of algae low even in slow moving water.

Which of the following, if true, does most to explain the contrast described above?

(A) During periods of extreme drought, the populations of some of the species that feed on algae tend to fall.

Irrelevant.

(B) The more slowly water moves, the more conducive its temperature is to the growth of algae.

Irrelevant.

(C) When algae populations reach very high levels, conditions within the river can become toxic for some of the other species that normally live there.

Irrelevant.

(D) Australian rivers dry up completely for short intervals in periods of extreme drought.

If rivers dry up completely for short periods , then there must be increased algae population during that period.

(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.

Out of scope and irrelevant.

Less rainfall --> Water level drops + rivers slow more slowly --> Algae population increases
Amount of algae per unit of water increases when there is little rain

Conclusion: After a period of extreme drought, algae levels are low even in slow moving water.

Possible explanation: Algae require atleast some quantity of water to thrive.

(A) During periods of extreme drought, the populations of some of the species that feed on algae tend to fall. - Incorrect - Opposite. Does not help resolve the paradox.

(B) The more slowly water moves, the more conducive its temperature is to the growth of algae. - Incorrect - States one more fact but does not answer the conclusion.

(C) When algae populations reach very high levels, conditions within the river can become toxic for some of the other species that normally live there. - Incorrect - Does not help resolve the paradox.

(D) Australian rivers dry up completely for short intervals in periods of extreme drought. - Correct - Since the rivers completely dry out algae cannot thrive in such conditions.

(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. - Incorrect - Out of context.

Answer: D

When there is less rainfall than normal, the water level of Australian rivers falls and the rivers flow more slowly. Because algae whose habitat is river water grow best in slow-moving water, the amount of algae per unit of water generally increases when there has been little rain. By contrast, however, following a period of extreme drought, algae levels are low even in very slow-moving river water.

Which of the following, if true, does most to explain the contrast described above?

(A) During periods of extreme drought, the populations of some of the species that feed on algae tend to fall.
-- This is strengthening the situation for more algae to be there in drought period

(B) The more slowly water moves, the more conducive its temperature is to the growth of algae.
-- This is again saying algae level to be higher against the situation
(C) When algae populations reach very high levels, conditions within the river can become toxic for some of the other species that normally live there.
-- Out of Scope
(D) Australian rivers dry up completely for short intervals in periods of extreme drought.
-- Best possible reason
(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.
-- Out of scope

While D is clearly the best answer of the available choices, this answer doesn't logically explain the paradox.

Sure, if a drought is so bad that there is no water at all, there won't be any algae.

However, the question asks us to explain the contrast between higher levels of algae in slow-moving rivers and lower levels in very slow-moving rivers. Here, we are (or should be) looking for a reason to explain why algae levels might not be inversely correlated with river speeds in all cases, as one would expect. The fact that rivers sometimes dry up completely doesn't address this contrast since in that case, the rivers aren't moving at all. We're told the algae's habitat is river water, so why do we care about when there isn't any river water at all? It's not relevant.


This seems like a prime example of having to select the best answer, not the ideal answer. I'm absolutely overthinking this one. Sorry, all!

Not really. The correct option makes complete sense. You missed out one word in the argument - "following" :
By contrast, however, following a period of extreme drought, algae levels are low even in very slow-moving river water.

So if there has been a period of extreme draught in the recent past (but is now over), algae levels are low even in very slow moving river water because the river had dried up completely during the draught. Obviously, that would have killed algae too since its habitat would have gone. So now, even though the water is back and is very slow moving, the algae population is low.

When there is less rainfall than normal, the water level of Australian rivers falls and the rivers flow more slowly. Because algae whose habitat is river water grow best in slow-moving water, the amount of algae per unit of water generally increases when there has been little rain. By contrast, however, following a period of extreme drought, algae levels are low even in very slow-moving river water.

Which of the following, if true, does most to explain the contrast described above?

(A) During periods of extreme drought, the populations of some of the species that feed on algae tend to fall.
--> This option seems to exacerbate the contrast.

(B) The more slowly water moves, the more conducive its temperature is to the growth of algae.
--> This option seems to exacerbate the contrast.

(C) When algae populations reach very high levels, conditions within the river can become toxic for some of the other species that normally live there.
--> This option does nothing to explain the contrast.

(D) Australian rivers dry up completely for short intervals in periods of extreme drought.
--> correct.

(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.
--> irrelevant information.

i chose A, BUT I have no clarity on this. please explain

______________________________
Have you read the discussion above?

I did it right but actually it seems not to explain well the situation since it is does not state anything about algae during extreme drought.

Gist of passage:

Algea level is low even when water flow is slow
Given the fact that algae level per unit water increase in slow water flow...

It is clear to find gap ...algae level per unit water is low and conclusion is about algae level is low so ofcourse drought have made water Quantities extremely low..

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Less Rainfall ----> Water Level of Australia falls ------> Rivers flow slowly ----- > Increase in amount of algae per unit area.

Extreme drought ------- > Level of algae low even in slow moving water.

Which of the following, if true, does most to explain the contrast described above?

(A) During periods of extreme drought, the populations of some of the species that feed on algae tend to fall.

Irrelevant.

(B) The more slowly water moves, the more conducive its temperature is to the growth of algae.

Irrelevant.

(C) When algae populations reach very high levels, conditions within the river can become toxic for some of the other species that normally live there.

Irrelevant.

(D) Australian rivers dry up completely for short intervals in periods of extreme drought.

If rivers dry up completely for short periods , then there must be increased algae population during that period.

(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.

Out of scope and irrelevant.

"Because algae whose habitat is river water" is mentioned in the passage.. so during drought, no water means no habitat for the algae..

hence D is the answer

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.

When there is less rainfall than normal, the water level of Australian rivers falls and the rivers flow more slowly. Because algae whose habitat is river water grow best in slow-moving water, the amount of algae per unit of water generally increases when there has been little rain. By contrast, however, following a period of extreme drought, algae levels are low even in very slow-moving river water.

Which of the following, if true, does most to explain the contrast described above?

(A) During periods of extreme drought, the populations of some of the species that feed on algae tend to fall. X
-Goes in the opposite direction…a decline in the population of species that feed on algae should mean that algae levels go up…but that’s not what we are seeing

(B) The more slowly water moves, the more conducive its temperature is to the growth of algae. X
-Again goes in the opposite direction

(C) When algae populations reach very high levels, conditions within the river can become toxic for some of the other species that normally live there. X
-Effect on other species is irrelevant…Why are the algae levels LOW?

(D) Australian rivers dry up completely for short intervals in periods of extreme drought.
Correct…well generally speaking algae needs water to survive…no water = no survival

(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. X
-Sidesteps the issue at hand…this choice does not explain why periods of extreme drought cause algae to be very low…so knowing that algae tends to be higher in rivers controlled by dammning is not helpful

When there is less rainfall than normal, the water level of Australian rivers falls and the rivers flow more slowly. Because algae whose habitat is river water grow best in slow-moving water, the amount of algae per unit of water generally increases when there has been little rain. By contrast, however, following a period of extreme drought, algae levels are low even in very slow-moving river water.

Which of the following, if true, does most to explain the contrast described above?

P : Algae best lives in slowly moving water
P : after extreme drought, when river water is moving very slowly, algae levels drop.

Why? Any reason to understand why extreme drought is pugnacious to algae will explain.

(A) During periods of extreme drought, the populations of some of the species that feed on algae tend to fall.
-> if the population that feed on algae drops, then it is reasonable to think that the population of algae will increase.

(B) The more slowly water moves, the more conducive its temperature is to the growth of algae.
-> Same as option A. If the water moves more slowly after extreme drought and such condition is conducive for algae to live, then the population of algae must increase.

(C) When algae populations reach very high levels, conditions within the river can become toxic for some of the other species that normally live there.
-> The condition becoming toxic to other species to live in is not a concern in this question.

(D) Australian rivers dry up completely for short intervals in periods of extreme drought.
-> Correct, Because rivers dry up completely in extreme drought, leaving less water for algae as a sustenance, algae population will drop.

(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.
-> Irrelevant. Condition within extreme drought is not our concern. We are only concerned about what would have caused the population of algae to drop.