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Register now and save up to $200 Available with Beat the GMAT members only code • 5 Day FREE Trial Study Smarter, Not Harder Available with Beat the GMAT members only code • Free Practice Test & Review How would you score if you took the GMAT Available with Beat the GMAT members only code • Get 300+ Practice Questions 25 Video lessons and 6 Webinars for FREE Available with Beat the GMAT members only code • Magoosh Study with Magoosh GMAT prep Available with Beat the GMAT members only code ## When there is less rainfall tagged by: ceilidh.erickson This topic has 5 expert replies and 5 member replies Amadalia Senior | Next Rank: 100 Posts Joined 20 Jan 2014 Posted: 76 messages Upvotes: 3 #### When there is less rainfall Sun Apr 20, 2014 8:59 am 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. the OA is D but I'm not really convinced why? "By contrast, however, following a period of extreme drought, algae levels are low even in very slow-moving river water[/u]. this sentence makes me think that the presence of water is necessary and answerD is completly out of scope. the answer Emakes sens as it said that in period of [u]extreme drought the algae prefer the flow freely river!!! Many thanks in advance!!!! gocoder Master | Next Rank: 500 Posts Joined 05 Dec 2015 Posted: 120 messages Target GMAT Score: 720 Sat Oct 14, 2017 4:40 am ceilidh.erickson wrote: (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. Any answer that starts "except during extreme drought" can't possibly tell us what it is about extreme drought that makes things different. This also just supports the given premise: slow rivers = high algae. It does nothing to explain why extreme drought = low algae. The contrast is well shown between the times of extreme drought and that of others. It suggests to me that E is wrong because this choice doesn't discuss about the Australian rivers in general; instead, it hinges on rivers are that dammed. gocoder Master | Next Rank: 500 Posts Joined 05 Dec 2015 Posted: 120 messages Target GMAT Score: 720 Sat Oct 14, 2017 4:40 am ceilidh.erickson wrote: (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. Any answer that starts "except during extreme drought" can't possibly tell us what it is about extreme drought that makes things different. This also just supports the given premise: slow rivers = high algae. It does nothing to explain why extreme drought = low algae. The contrast is well shown between the times of extreme drought and that of others. It suggests to me that E is wrong because this choice doesn't discuss about the Australian rivers in general; instead, it hinges on rivers are that dammed. Is this a valid reasoning...? bubbliiiiiiii Legendary Member Joined 14 Apr 2009 Posted: 971 messages Followed by: 12 members Upvotes: 49 Test Date: 18.10.2012 Target GMAT Score: 760 GMAT Score: 700 Mon Apr 21, 2014 2:01 am The question stem states that: During low rain than normal the water flow is slow, stimulating algae growth. However, following extreme drought algae growth is low even in slow moving river water. To resolve the contrast, there should be some additional requirement, besides slow moving water, for algae growth. That could be minerals from fresh rain water, appropriate water level, even in slow moving waters, etc. Option D hits the contrast by pointing completely dry rivers, hinting to low water levels. Why not E: E talks about the period EXCEPT during drought, a period irrelevant to our discussion. Hope it helps. _________________ Regards, Pranay ### GMAT/MBA Expert ceilidh.erickson GMAT Instructor Joined 04 Dec 2012 Posted: 1778 messages Followed by: 228 members Upvotes: 1443 Mon Apr 21, 2014 7:27 am When you have a CR problem that asks you to EXPLAIN A DISCREPANCY, it's usually in this format: It's generally true that ______. Normally, you'd think this would mean ______. In this case, though, _______. In this case: It's generally true that less rainfall = slow rivers, and slow river = algae. Normally, you'd think this would mean that drought = more algae. In this case, though, extreme drought = low algae levels. We need to find an answer that 1) keeps the original conditions true, and 2) supports the surprising outcome. Avoid answer choices that try to undermine the premises or support the expected outcome. The key is to find an answer that addresses the difference that EXTREME drought makes versus normal drought conditions. (A) During periods of extreme drought, the populations of some of the species that feed on algae tend to fall. This supports the expected outcome - if algae-eaters decline, there should be MORE algae. (B) The more slowly water moves, the more conducive its temperature is to the growth of algae. This explains why the premise (slow water = high algae) is true, but doesn't explain the surprising outcome. (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. Other species are irrelevant (D) Australian rivers dry up completely for short intervals in periods of extreme drought. If the rivers dry up completely in EXTREME drought, then the rivers aren't just slow-moving - they don't exist anymore! The habitat that the algae lives in ceases to exist, so much of the algae would die. This keeps the original conditions true, and explains the surprising answer. 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. Any answer that starts "except during extreme drought" can't possibly tell us what it is about extreme drought that makes things different. This also just supports the given premise: slow rivers = high algae. It does nothing to explain why extreme drought = low algae. To your question: Quote: "By contrast, however, following a period of extreme drought, algae levels are low even in very slow-moving river water[/u]. this sentence makes me think that the presence of water is necessary and answerD is completly out of scope. You're absolutely right! The presence of water is necessary for algae growth - but that actually makes D totally relevant, not out of scope. Generally speaking, we don't want to think of "scope" the same way on EXPLAIN DISCREPANCY questions as we do on other CR questions. The right answer will always be a new piece of information, and it's ok if it feels out-of-the-blue. It just has to keep the original conditions and the surprising outcome true. _________________ Ceilidh Erickson Manhattan Prep GMAT & GRE instructor EdM in Mind, Brain, and Education Harvard Graduate School of Education Manhattan Prep instructors all have 99th+ percentile scores and expert teaching experience. Sign up for a FREE TRIAL, and learn why we have the highest ratings in the GMAT industry! Free Manhattan Prep online events - The first class of every online Manhattan Prep course is free. Classes start every week. ### GMAT/MBA Expert ceilidh.erickson GMAT Instructor Joined 04 Dec 2012 Posted: 1778 messages Followed by: 228 members Upvotes: 1443 Mon Apr 21, 2014 7:30 am _________________ Ceilidh Erickson Manhattan Prep GMAT & GRE instructor EdM in Mind, Brain, and Education Harvard Graduate School of Education Manhattan Prep instructors all have 99th+ percentile scores and expert teaching experience. Sign up for a FREE TRIAL, and learn why we have the highest ratings in the GMAT industry! Free Manhattan Prep online events - The first class of every online Manhattan Prep course is free. Classes start every week. ### GMAT/MBA Expert David@VeritasPrep GMAT Instructor Joined 22 Feb 2010 Posted: 2193 messages Followed by: 508 members Upvotes: 1186 GMAT Score: 770 Tue Apr 22, 2014 11:21 am Amadalia - Others have already discussed this question, but I just wanted to point out the two most important words in the question: "short intervals" Choice D says: (D) Australian rivers dry up completely for short intervals in periods of extreme drought. I see that what you are thinking is - if the river is dry then there is no water! But what D is saying is not that the river stays dry, but that most of the time there is water and sometimes for brief periods the river dries up completely. It must be that this kills the algae. It is like saying that working women earn less than men in the U.S. And the answer choice says "Women often leave the workforce for SHORT INTERVALS." So these are still working women even if they are not employed for a few months here or there...so it is not out of scope and maybe leaving the workforce for even a short time sort of stalls or resets your career. So that would explain. Hope it helps! _________________ Veritas Prep | GMAT Instructor Veritas Prep Reviews Save$100 off any live Veritas Prep GMAT Course

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Kamal2014 Junior | Next Rank: 30 Posts
Joined
12 May 2014
Posted:
22 messages
GMAT Score:
730
Tue May 13, 2014 5:55 am
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.

the OA is D but I'm not really convinced why?
"By contrast, however, following a period of extreme drought, algae levels are low even in very slow-moving river water[/u].
this sentence makes me think that the presence of water is necessary and answerD is completly out of scope.
the answer Emakes sens as it said that in period of [u]extreme drought
the algae prefer the flow freely river!!!
Hi
The answer for this question id D

Reason: The reason is simple. Just look at the second sentencer. It says that the habitat for the algae is river water. If the rivers dry up then there is no availability of the habitat. Hence drying up of rivers decreases algae population. Hope it helps
-------------
Inner peace

gocoder Master | Next Rank: 500 Posts
Joined
05 Dec 2015
Posted:
120 messages
Target GMAT Score:
720
Thu Sep 21, 2017 1:22 am
ceilidh.erickson wrote:
(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.
Other species are irrelevant

If the choice says the following, will it right ?
" When algae populations reach very high levels, conditions within the river can become toxic for some of the species of algae that normally live there.

### GMAT/MBA Expert

ceilidh.erickson GMAT Instructor
Joined
04 Dec 2012
Posted:
1778 messages
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228 members
1443
Wed Oct 25, 2017 2:05 pm
gocoder wrote:
ceilidh.erickson wrote:
(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.
Other species are irrelevant

If the choice says the following, will it right ?
" When algae populations reach very high levels, conditions within the river can become toxic for some of the species of algae that normally live there.
No, this wouldn't be enough to make this a right answer. For one thing, "lots of algae = toxic conditions for algae" doesn't necessarily mean we'd get back down to LOW algae levels. It would just mean that very high levels of algae are not sustainable.

For another, "some species of algae" is not comprehensive enough. If the prompt talks about all algae growth in general, specifying that something affects only "some" algae would not be enough. Be wary of answer choices that create subsets within a category when we're talking about the category as a whole.

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### GMAT/MBA Expert

ceilidh.erickson GMAT Instructor
Joined
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Posted:
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Wed Oct 25, 2017 2:08 pm
gocoder wrote:
ceilidh.erickson wrote:
(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.
Any answer that starts "except during extreme drought" can't possibly tell us what it is about extreme drought that makes things different. This also just supports the given premise: slow rivers = high algae. It does nothing to explain why extreme drought = low algae.

The contrast is well shown between the times of extreme drought and that of others. It suggests to me that E is wrong because this choice doesn't discuss about the Australian rivers in general; instead, it hinges on rivers are that dammed.

Is this a valid reasoning...?
Definitely valid! This answer choice gives further support to the idea that slow-moving water --> algae (if we use outside knowledge to infer that damming rivers makes them flow more slowly). But it does nothing to explain the contrast between extreme drought and low rainfall.

_________________

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Manhattan Prep GMAT & GRE instructor
EdM in Mind, Brain, and Education

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