Brown tides are growths of algae on the sea's surface that prevent sun

Question

Brown tides are growths of algae on the sea's surface that prevent sunlight from reaching marine plants below, thereby destroying not only the plants but also the shellfish that live off these plants. Biologists recently isolated a virus that, when added to seawater, kills the algae that cause brown tides. Adding large quantities of this virus to waters affected by brown tides will therefore make it possible to save the populations of shellfish that inhabit those waters.

Which of the following, if true, provides the most support for the conclusion of the argument?

A. When applied in large quantities, the virus not only kills the algae that cause brown tides but also many harmless kinds of algae.

B. Marine animals that prey on shellfish avoid areas of the sea in which brown tides are occurring.

C. The number of different kinds of virus present in seawater is far greater than many marine biologists had, until recently, believed.

D. The presence of large quantities of the virus in seawater does not adversely affect the growth of marine plants.

E. The amount of the virus naturally present in seawater in which brown tides occur is neither significantly greater nor significantly less than the amount present in seawater in which brown tides do not occur.

CR41700.02

Krishh9119 · Accepted Answer

Question Type: Strengthen

Conclusion: Adding large quantities of this virus in brown tides -> will save the populations of shellfish in brown tides

What did we learn:
1) what saves the shellfish population:  Marine plants 
2) what is harmful for shellfish population:  brown tides 
 
Pre-thinking:  
1)  To save the populations of shellfish , we need something that  kills  the brown tides but that does  not adversely  effect the Marine plants.

Option D: The presence of large quantities of the virus in seawater does not adversely affect the growth of marine plants.
This option clearly falls with our reasoning.

D is our answer.

HaileyCusimano · Answer

Certainly, vasuca10 - happy to provide my thoughts! If we get started by taking a look at the question stem: We can see that we're looking at a strengthen question. So, we need something that fortifies the connection between evidence and conclusion. In this case, we want something that strengthens the use of the facts we've been given: and to conclude that: Let's take a look at our answers! This answer doesn't strengthen the force of our evidence. Knowing that the virus kills other varieties of algae certainly doesn't help reinforce that it could be used to save populations of shellfish. If anything, the virus holds the potential to disturb their ecosystem if it kills other, harmless, varieties of algae. This one's out. Hmm.. if the predators of shellfish avoid areas of brown tides, and we're proposing a plan of action that would get rid of brown tides, all this tells us is that the shellfish could encounter a new set of problems (those darn predators!) if the brown tides were effectively eliminated. This doesn't strengthen the force of the evidence, it weakens it! Totally irrelevant to us here. We're suggesting adding large quantities of this virus that we already know has been seen to kill brown-tide-causing algae. So, the fact that there are different varieties does not matter at all to us here, as it does not impact the connection between evidence and conclusion. (Does it make sense to use the virus accomplish the goal of saving shellfish?) Hey! This looks like the counter-part to what I mentioned in (A). If the shellfish live off plants, and we want to strengthen the use of the virus to save populations of shellfish, this answer option tells us that not only will the shellfish avoid destruction by brown tides, its ecosystem and food source will as well! This absolutely strengthens our plan of action and corresponding conclusion. Naturally present quantities don't matter to use here... our argument specifically notes that the addition of large quantities of the virus will impact the brown tides and thus, the shellfish population. So, the naturally present levels do not matter to us, as they fall outside of the realm of the argument made. It sounds like you analyzed the impact of (B) correctly, but misidentified the objective of our answer. Here, we wanted something that strengthens the potential use of this virus to save shellfish populations. (B), even in the light you described it in, weakens this argument, as it gives us reason to believe the use of the virus might not save the shellfish, and could instead just expose them to a whole new heightened threat! (poor shellfish!) So, be sure you're identifying your task or job with the question and how wrong answers impact the argument in the wrong way. This should help you recognize these wrong answer types more consistently moving forward!

I hope this helps!

adkikani · Answer

GMATCoachBen   MentorTutoring   VeritasKarishma

Can you help me with correct reasoning for B and D?

I need to find an evident that makes claim more believable.

Conclusion marker: Therefore. 
It is possible to save the populations of shellfish that inhabit those waters.
Premise: 
Adding large quantities of virus to waters kills algae.

What else I know:
Algae feed on plants on which shellfish feed.
So I if I kill algae with virus, then there will be more food for shellfish to survive on, and hence we can save more of shellfish.

Can I rephrase this as: Marine animals that prey on shellfish shall go to areas of the sea in which brown tides are absent.
Now if virus kills algae and then the animals that prey on shellfish will be more, decreasing shellfish population.
This is opposite to what I am looking for. 
Please help if there is better way of approach for this choice.

I guess I ran swiftly through:  Adding large quantities  in premise that made me ignore this choice.
However what this choice does is to prevent the claim from being disrupted (i.e. why shellfish will survive) IN SPITE OF
using large quantities that will kill algae but not affect plants on which shellfish feed on.
Am I correct?

AndrewN · Answer

Hello,  adkikani . There are a few points in your post that I would like to touch on (below).

Fine up to this point.

Careful. This part is NOT something you can deduce from the passage, which says nothing about the feeding habits of algae. That is, we are not told that algae compete with shellfish for food, but that growths of algae on the surface of the water block sunlight, thereby killing marine plants under the surface that depend on that sunlight. This type of fine point might not make a difference in this question, but it certainly could in another CR question. Misinterpretations or leaps in logic cause people to miss CR questions with a higher frequency. Just stick to exactly what the passage says, and you will be much less likely to get creative.

No, you understood this weakener perfectly. We are not trying to show how the shellfish population would suffer if the brown tides were dispersed, but to show how the population would be saved even after brown tides had been present.

Yes, this is another correct interpretation. Do not be afraid to slow your approach somewhat to process how the answer choices align or do not align with the information presented in the passage. Accuracy first. Speed can follow.

Good luck with your studies, and thank you for  having tagged  me (to be a little more proper, and in keeping with our Michelangelo dialogue from yesterday).

- Andrew

NiftyNiffler · Answer

Brown tides are growths of algae on the sea's surface that prevent sunlight from reaching marine plants below, thereby destroying not only the plants but also the shellfish that live off these plants. Biologists recently isolated a virus that, when added to seawater, kills the algae that cause brown tides. Adding large quantities of this virus to waters affected by brown tides will therefore make it possible to save the populations of shellfish that inhabit those waters.

Which of the following, if true, provides the most support for the conclusion of the argument?

A. When applied in large quantities, the virus not only kills the algae that cause brown tides but also many harmless kinds of algae.  -- Weakening

B. Marine animals that prey on shellfish avoid areas of the sea in which brown tides are occurring.  -- Doesn't matter since the shellfish anyways get killed by the brown tides algae
 
C. The number of different kinds of virus present in seawater is far greater than many marine biologists had, until recently, believed.  -- Out-of-scope. The current virus presence is not at issue here
 
D. The presence of large quantities of the virus in seawater does not adversely affect the growth of marine plants.  -- CORRECT, this answer option says that other than killing brown tide algae, the virus won't cause any harm
 
E. The amount of the virus naturally present in seawater in which brown tides occur is neither significantly greater nor significantly less than the amount present in seawater in which brown tides do not occur.  -- This speaks nothing about the introduction of the particular algae killing virus species. IRRELEVANT

So, answer is  D

vasuca10 · Answer

VeritasPrepHailey  Mam Kindly guide for this question...I marked answer choice B as I thought that if virus kills algae and then the animals that prey on shellfish will be more, decreasing shellfish population.

vasuca10 · Answer

parkhydel  Buddy Please correct the question type tag also its a strengthen question and not an inference question

vasuca10 · Answer

BLTN · Answer

I do like following OG answer:

A. When applied in large quantities, the virus not only kills the algae ~~that cause brown tides~~ but also many harmless kinds of algae.

But also what ? Salvage, destroy, feed ?

AndrewN · Answer

Wow, talk about a facepalm moment. Nice catch,  Mike20201 . That  kills  should definitely precede the  not only . I guess this is a case in which one hand did not talk to the other, and a grammatical error appeared in the OE of an SC question of all places.   Edit  : Just realized you were not referencing the OE, but simply the answer choice itself. Anyway, still a good catch, a subtlety that tests knowledge of an idiomatic construct and the meaning it conveys.

Thank you for pointing it out. Others can appreciate the distinction, and more and more of us will feel in on the joke.

- Andrew

mSKR · Answer

Hi  AndrewN  sir,

A. When applied in large quantities, the virus not only kills the algae that cause brown tides  but also many harmless kinds of algae.

What's your intake on A?
I understand 2nd part( red  part) doesn't add force to our strengthening claim especially when D is present that directly impacts our conclusion. Undoubtedly D is best answer here.

What would you suggest in general- to keep such options on hold in 1st reading , avoiding selection and striking off?
in Official GMAT world, such options would rarely be correct ( based on your experience ) , or we never know such option could be only best option?

Please give your suggestions.

Thanks!

AndrewN · Answer

Hello,  mSKR . The first part of (A) checks out against the information in the last two lines of the passage. Of course, the problem starts with  but , but if the first part looked good enough to you and you were uncertain about the latter part, sure, you could leave (A) alone while you checked other answers for more glaring problems. It is only in a subsequent pass of the answer choices that you would need to get into the finer details, provided you had not burned off four answers to begin with. Such an approach can help you work through questions efficiently, with an added bonus that you may also find that your confidence increases. (Rather than spend a lot of time qualifying or disqualifying each answer choice, one by one, you can burn off what you know is incorrect and then work with the rest, putting less stress on your mind.)

- Andrew

CrackverbalGMAT · Answer

Let’s look at the premises and the conclusion of the given argument.

Premises

Brown tides are growths of algae on the sea's surface that prevent sunlight from reaching marine plants below, thereby destroying not only the plants but also the shellfish that live off these plants.
Biologists recently isolated a virus that, when added to seawater, kills the algae that cause brown tides.

Conclusion

Adding large quantities of this virus to waters affected by brown tides will therefore make it possible to save the populations of shellfish that inhabit those waters.

We need to find an option that supports the conclusion.

A. When applied in large quantities, the virus not only kills the algae that cause brown tides but also many harmless kinds of algae.
Option A tells why adding large quantities of this virus can be problematic- because it kills many harmless kinds of algae too. Definitely doesn’t support the conclusion.  Eliminate .

B. Marine animals that prey on shellfish avoid areas of the sea in which brown tides are occurring.
The conclusion is limited to - Adding large quantities of this virus to waters affected by brown tides make it possible to save the populations of shellfish that inhabit those waters. Option B is irrelevant to the conclusion.  Eliminate .

C. The number of different kinds of virus present in seawater is far greater than many marine biologists had, until recently, believed.
Again, doesn’t support the conclusion. Irrelevant.  Eliminate .

D. The presence of large quantities of the virus in seawater does not adversely affect the growth of marine plants.
The stimulus says- 
The growth of algae blocks sunlight from reaching marine plants. Shellfish feed on marine plants. Plan- Add virus to kill the algae that cause brown tides
 
If the presence of large quantities of the virus in seawater does not negatively affect the growth of marine plants, then we have all the more reasons to add large quantities of this virus to waters to save the populations of shellfish that inhabit those waters.
 Correct .

E. The amount of the virus naturally present in seawater in which brown tides occur is neither significantly greater nor significantly less than the amount present in seawater in which brown tides do not occur.
Irrelevant.  Eliminate .

Vishnupriya
GMAT Verbal SME

singhall · Answer

GMATCoachBen   MentorTutoring   VeritasKarishma   GMATNinja

Hello Experts!

I have a confusion between A and D.

A says When applied in large quantities, the virus not only kills the algae that cause brown tides  but also many harmless kinds of algae. 
From the discussion on the forum, I have understood that we want to point out that this "harmless" algae could still be important for the ecosystem of the plants on which shellfish survive, or the shellfish itself. OKAY!  but this is an assumption we have made to strike off this answer choice, it is not mentioned anywhere in the premise or the answer choice.

B The presence of large quantities of the virus in seawater   does not adversely affect   the growth of marine plants.
How come this is acceptable? This answer choice clearly says it doesn't adversely affect the growth of marine plants, adversely is not too much damage or something we can't really quantify. But generally it is talking about damage (may be a 10% damage). This answer choice is clearly saying there is damage on marine plants while in A we are assuming there could be damage. Then how is this better over A?

Please help with this.

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Brown tides are growths of algae on the sea's surface that prevent sun

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