In Australia, in years with below-average rainfall, less water goes in

Question

In Australia, in years with below-average rainfall, less water goes into rivers and more water is extracted from rivers for drinking and irrigation. Consequently, in such years, water levels drop considerably and the rivers flow more slowly. Because algae grow better the more slowly the water in which they are growing moves, such years are generally beneficial to populations of algae. But, by contrast, populations of algae drop in periods of extreme drought.

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

(A) Algae grow better in ponds and lakes than in rivers.

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

(C) Algae cannot survive in the absence of water.

(D) Algae must be filtered out of water before it can be used for drinking.

(E) The larger the population of algae in a body of water, the less sunlight reaches below the surface of the water.

ID: 500253

Kurtosis · Accepted Answer

Question Type: Explain/Paradox

Argument: Algae growth is better when river water flows slowly but in periods of extreme drought populations of algae drop.

(A) Algae grow better in ponds and lakes than in rivers. - Irrelevant

(B) The more slowly water moves, the more conducive its temperature is to the growth of algae. - While mentioning about the drop in algae population, the argument focuses on periods of extreme drought. This option doesn't explain the paradox.

(C) Algae cannot survive in the absence of water. - Correct.

(D) Algae must be filtered out of water before it can be used for drinking. - Irrelevat

(E) The larger the population of algae in a body of water, the less sunlight reaches below the surface of the water. - Irrelevant.

Answer: C

Canteenbottle · Answer

C can't be right. No where in the question says extreme drought means absence of water. In fact, extreme drought just means more water will be extracted from the river.

pikolo2510 · Answer

Hey

The premise doesn't anywhere mention that extreme drought means 'more water will be extracted from the river' . This statement is assumption that you think

also, this question is an 'resolve the paradox' kind of question. So external ideas are allowed

Only C answers the question, rest are all out of scope

Canteenbottle · Answer

"in years with below-average rainfall, less water goes into rivers and more water is extracted from rivers for drinking and irrigation." this is the info from the question. It is not an assumption I made. No where in the question suggests absence of water. Lack of water and absence of water are two different concepts.

pikolo2510 · Answer

Hey  Canteenbottle  (BTW, nice name)

I am only referring to the statement in the previous comment "extreme drought just means more water will be extracted from the river"

We don't see any such statement or reference in the premise. Anyways, you are absolutely spot on here. Lack of water and absence of water are two different concepts and tthiss tells us why C is the right choice

Lack of water (rivers flow slowly) - algae growth is much better
Absence of water (drought) - No water at all, how will the algae grow? Hence populations of algae drop in periods of extreme drought

hence answer is C. Hope this helps

gvij2017 · Answer

A, D and E are clearly out.

(B) The more slowly water moves, the more conducive its temperature is to the growth of algae.
This doesn't tell why algae population decrease, So it is not useful choice here.

(C) Algae cannot survive in the absence of water.
Water is essential for algae formation. According to stimulus, water level is low and flow is quite slow in drought condition.

So C is winning choice to resolve the paradox.

GMATNinjaTwo · Answer

Thanks  pikolo2510  and  gvij2017  for your explanations!

To post additional questions not already addressed in this thread, feel free to use the  request verbal experts' reply  button.

blitzkriegxX · Answer

Resolve the paradox -
In such type of questions we can bring in outside or new information.

Read slowly and observe carefully the last line "But, by contrast, populations of algae drop in periods of extreme drought."
Though the argument mostly talks about slow water flow during low level rainfall and its benefit to algae, in the last line it tries to bring in a contrast that happens in extreme drought. So we need to reason why algae is less in extreme drought. Making option C correct.

Also this shows how important POE is on verbal. If u use POE u will be left with C and even if you miss the details in last sentence, you should be able to arrive at C with just POE.

Hope it helps!

Andrewcoleman · Answer

There are three levels of progression in this logic train.

DOWN in rain > DOWN in River levels and UP Irrigation
Which leads to

DOWN in river levels > DOWN in river velocity
Which leads to UP in algae

Would it be advisable to skip the middle steps mentally and go straight for the end result.

I feel like I wasted a minute just keeping track of the secondary and tertiary effects of the lower rain, when I didn't really need them.

In Australia, in years with below-average rainfall, less water goes into rivers and more water is extracted from rivers for drinking and irrigation.

Consequently, in such years, water levels drop considerably and the rivers flow more slowly.

Because algae grow better the more slowly the water in which they are growing moves, such years are generally beneficial to populations of algae.??

AndrewN · Answer

Hello,  Andrewcoleman . I understand your frustration with CR questions. I used to feel the same way, falling into logic traps left and right. (Well, I was decent, but nowhere near as good as I was on other types of questions.) I would not advise ignoring any part of the passage, since you can never be sure what to expect of the answer choices and from which parts of the passage you may need to draw. One approach I learned the hard way that I have found much success with is reading the question first, before looking at anything in the passage. That way, I go in kind of knowing what to look for, and my pre-thinking cylinders are firing. Some time ago, I wrote  another post  on the general topic of CR questions, and this particular question, with an out-of-left-field answer, reminded me of one I wrote about in that post.

In this question, you are expecting all that information about slow-moving and fast-moving water to add up to something, but  drought , a period of dryness, might lead to no water at all, especially if it is  extreme , and if algae depend on at least  some  water to survive, then the  extreme drought  of the final line of the passage could be pointed to to explain why the algae that would typically thrive in slow-moving water would not do so under these particular weather conditions. Thus, the  contrast  or paradox of the question stem is resolved.

I would be happy to answer any other questions you may have. Good luck with your studies.

- Andrew

mSKR · Answer

Why D can not be answer . please see the below explanation.

drought means low rainfall for long period of time. it means the water level is too low. 
As more water is extracted from rivers for drinking and irrigation it means the quantity of algae is reduced from water( because now more drinking and irrigation water would have high density of algae) , AND then Algae is filtered out of water before it can be used for drinking,

As drought increase, more algae is extracted in drinking water and hence more algae is thrown out which leads to less quantity of algae in the water.

GMATNinja ,  GMATNinjaTwo ,  nightblade354 ,  mira93 ,  eakabuah

nightblade354 · Answer

In Australia, in years with below-average rainfall, less water goes into rivers and more water is extracted from rivers for drinking and irrigation. Consequently, in such years, water levels drop considerably and the rivers flow more slowly. Because algae grow better the more slowly the water in which they are growing moves, such years are generally beneficial to populations of algae. But, by contrast, populations of algae drop in periods of extreme drought.

P: In Australia, in years with below-average rainfall, less water goes into rivers and more water is extracted from rivers for drinking and irrigation 
P: Consequently, in such years, water levels drop considerably and the rivers flow more slowly 
P: Because algae grow better the more slowly the water in which they are growing moves, such years are generally beneficial to populations of algae
C: But, by contrast, populations of algae drop in periods of extreme drought

Long set up, but simple structure: less rains fall means less river water --> water levels drop --> water moves slower --> should be perfect for algae --> BUT, the opposite occurs. How do we resolve the fact that conditions are perfect, but the opposite occurred? There are too many reason to think, but this is what we need to think about as we go through each answer choice.

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

(A) Algae grow better in ponds and lakes than in rivers. --  Who cares? We are only concerned with growth rate in rivers; this is an irrelevant comparison because rivers and lakes are not the same. Out.

(B) The more slowly water moves, the more conducive its temperature is to the growth of algae. --  OK, so this strengthens the idea that the algae should grow faster. But again, our algae does the opposite. So this is like an additional premise, but it still has no impact on our argument. Out. 
 
(C) Algae cannot survive in the absence of water. --  Interesting. If the water is completely gone, the algae cannot survive. Hold this on first pass and come back to it. And now that you've come back, this is our answer. I will admit I dislike it, but the rest are so out of scope that this has to be it. If there is no water, which is a negative, then the algae can't grow and that helps resolve our issue. The river goes too far in the direction the algae need and instead becomes unsuitable for growth. I dislike that we have to assume in this instance, for the argument, that the river becomes dry, but it is what it is.

(D) Algae must be filtered out of water before it can be used for drinking. --  OK, but who cares? We are told that algae has to be taken out before the water can be used. But what does this have to do with our argument? If you guessed this, you were assuming that in years with more water that rivers aren't used as much for drinking water. But water is used no matter what, so the algae is filtered out no matter what. Further, how much is filtered out? Maybe no algae makes its way into the water, but the algae is poisonous so we have to filter it out if it gets in? Maybe it's one strand a year and because the algae is strong it never breaks away. Both these situations would have no impact on the population. And because we cannot make any assumptions, this is out.

(E) The larger the population of algae in a body of water, the less sunlight reaches below the surface of the water. --  But we do not care about sunlight. Where are we told about algae growth compared to sunlight? And where are we told that too much algae can hurt their growth? It is never explained, so we cannot use it. Don't make any assumptions to justify this answer.

MHIKER · Answer

Drought means a period of dryness and one that causes extensive damage to crops or prevents their successful growth.

Thus, the answer is C

woohoo921 · Answer

I would be so appreciative if an expert can weigh in on my reasoning, as I was stuck between Choice B and C.

For Choice B, if a drought occurred, then water would move even more slowly = an even more conducive temperature for algae growth.

But this isn't the case. There is a level in which the algae just die off. So, although water that moves very slowly is ideal, algae can only survive up to the point in which there IS water.

RonTargetTestPrep · Answer

B is unhelpful because the passage text already tells us that algae grow better in slower-moving water. B offers only a possible reason WHY this might be true, by filling in the missing link of a specific mechanism whereby this correlation might operate.

In other words, the knowledge in the original text is
 slower-moving water   better algae growth

and B just fills in the missing step,
 slower-moving water —> better temperature for algae —> better algae growth

...which has no effect on the argument. Nothing depends on whether the correlation between slow water and algae growth works (or doesn't) by way of any specific mechanism, temperature or otherwise.

MUCH more importantly,  the main issue that NEEDS to be addressed—because the initial passage text presents NO REASON AT ALL for it—is the observation that, after growing more and more and more, the algae just VANISH once drought surpasses a certain intensity. 
B leaves this issue unaddressed, while instead providing superfluous explanation for a correlation whose existence is already established from the outset.

C, on the other hand, explains the sudden vanishing of the algae perfectly: It happens when drought conditions become intense enough to dry up ALL of the water for some period of time, instantly killing the algae (which live in water and therefore 'drown' without it).

VerbalBot · Answer

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In Australia, in years with below-average rainfall, less water goes in

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In Australia, in years with below-average rainfall, less water goes in

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