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

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When there is less rainfall than normal, the water level of [#permalink]

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02 Jul 2012, 08:14
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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.
(B) The more slowly water moves, the more conducive its temperature is to the growth of algae.
(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.
(D) Australian rivers dry up completely for short intervals in periods of extreme drought.
(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.

[Reveal] Spoiler:
Request you to please provide an explanation of A vs D.
Thanks
H
[Reveal] Spoiler: OA

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Re: When there is less rainfall than normal [#permalink]

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02 Jul 2012, 08:29
Less rainfall ---> slow movement of water ----> more algae
After the drought --->less water/flow----------->less algae

Answer must explain that even if the drought bring low water ,the growth of algae drops.

We need to prove that the growth of algae depends on other factors also OR the reason decremented algae is something else? OR there is no water at all for the growth of algae.

(D) says it clearly that "... rivers dry up completely for short intervals in periods of extreme drought.

Means-No water /or very less in a after-drought condition.
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Re: When there is less rainfall than normal [#permalink]

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02 Jul 2012, 09:09
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(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.
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Re: When there is less rainfall than normal, the water level of [#permalink]

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03 Jul 2012, 21:06
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imhimanshu wrote:
When there is less rainfall than normal, the water level of Australian rivers falls and the rivers fl ow 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.
(B) The more slowly water moves, the more conducive its temperature is to the growth of algae.
(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.
(D) Australian rivers dry up completely for short intervals in periods of extreme drought.
(E) Except during periods of extreme drought, algae levels tend to be higher in rivers in which the fl ow has
been controlled by damming than in rivers that fl ow freely.

Request you to please provide an explanation of A vs D.
Thanks
H

Responding to a pm:

This is an explain the paradox question. What is the paradox? When there is less rainfall, rivers move slowly and hence algae levels are high. When there is extremely less rainfall, algae levels are low even in very slow moving rivers.

You would expect algae levels to be very high in case of very slow moving rivers but it is not so. You need to explain why the algae levels are low in very slow moving rivers in case of drought.

Option (A) is incorrect. It says that during drought, some species that feed on algae fall. This means algae's predators are few and hence, algae should thrive and their levels should be high. This certainly doesn't explain why their levels fall in case of drought.

Option (D) explains the paradox. In case of drought, rivers actually dry up completely for short intervals. Hence, all algae in the river would die out if it dries up. Therefore, the algae population would be low.
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Re: When there is less rainfall than normal, the water level of [#permalink]

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19 Apr 2013, 06:31
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imhimanshu wrote:
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.
(B) The more slowly water moves, the more conducive its temperature is to the growth of algae.
(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.
(D) Australian rivers dry up completely for short intervals in periods of extreme drought.
(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.

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.

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Re: When there is less rainfall than normal, the water level of [#permalink]

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19 Apr 2013, 16:55
imhimanshu wrote:
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.
(B) The more slowly water moves, the more conducive its temperature is to the growth of algae.
(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.
(D) Australian rivers dry up completely for short intervals in periods of extreme drought.
(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.

[Reveal] Spoiler:
Request you to please provide an explanation of A vs D.
Thanks
H

A cannot be the answer. if populations of some of the species that feed on algae tend to fall, algae levels must increase. A is the opposite answer.
KEY here is algae grow best in slow-moving water, if there's no water, algae cannot grow. Hence, D is correct.

Hope it helps.
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Re: When there is less rainfall than normal, the water level of [#permalink]

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15 Feb 2014, 01:52
its D.. bridges the gap b/w two given situations..

algae population increases in drough condition

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

algae population didnt increase in extreme drought condition

what do happen in extreme drought condition for that algae population didnt increase..highlighted words fill the gap and resolve it !
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Re: When there is less rainfall than normal, the water level of [#permalink]

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15 Feb 2014, 10:55
imhimanshu wrote:
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.

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.
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Re: When there is less rainfall than normal, the water level of [#permalink]

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21 Jan 2016, 14:42
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Re: When there is less rainfall than normal, the water level of [#permalink]

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11 May 2016, 05:37
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.

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Re: When there is less rainfall than normal, the water level of [#permalink]

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19 Jun 2016, 01:56
I totally thought the drought killed algae because the rivers would dry up, not realizing that that the rivers would dry up is an assumption. I expected the correct answer choice to say that droughts kill algae. Close enough!
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Re: When there is less rainfall than normal, the water level of [#permalink]

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24 Jun 2016, 05:20
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
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Re: When there is less rainfall than normal, the water level of [#permalink]

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09 Mar 2017, 02:33
Question stem says : Algae grow more in slow moving water. In contrast, but after a drought the algae levels are low even in very slow-moving river water.

This brings us to an assumption that something is wrong with the slow moving water and hence algae level is low.
Which option will be apt? Only D makes sense. Rivers dry up and there is no slow moving water for algae to grow.
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Re: When there is less rainfall than normal, the water level of [#permalink]

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09 Mar 2017, 12:19
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.
(B) The more slowly water moves, the more conducive its temperature is to the growth of algae.
(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.
(D) Australian rivers dry up completely for short intervals in periods of extreme drought.
(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

A:other species is out of scope.
B:this does not resolve the paradox.
C:other secies is out of scope.
D:Correct answer,when the river dries up completely the algae also dies,and starts to grow from start once the water startes flowing.
E:out of scope. Also,Damming is limited

=======================
PreThinking:Resolve paradox question require one to find the connection between two opposite premises.
=======================
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Re: When there is less rainfall than normal, the water level of [#permalink]

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26 May 2017, 14:43
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!
Re: When there is less rainfall than normal, the water level of   [#permalink] 26 May 2017, 14:43
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