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To prevent a newly built dam on the Chiff River from blocking Updated on: 15 Oct 2024, 10:58
Originally posted by dgr8sandeep on 03 Sep 2015, 03:45.
Last edited by Bunuel on 15 Oct 2024, 10:58, edited 5 times in total.
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To prevent a newly built dam on the Chiff River from blocking the route of fish migrating to breeding grounds upstream, the dam includes a fish pass, a mechanism designed to allow fish through the dam. Before the construction of the dam and fish pass, several thousand fish a day swam upriver during spawning season. But in the first season after the project's completion, only 300 per day made the journey. Clearly, the fish pass is defective.

Which of the following, if true, most seriously weakens the argument?

(A) Fish that have migrated to the upstream breeding grounds do not return down the Chiff River again.

(B) On other rivers in the region, the construction of dams with fish passes has led to only small decreases in the number of fish migrating upstream.

(C) The construction of the dam stirred up potentially toxic river sediments that were carried downstream.

(D) Populations of migratory fish in the Chiff River have been declining slightly over the last 20 years.

(E) During spawning season, the dam releases sufficient water for migratory fish below the dam to swim upstream.­


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C
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To prevent a newly built dam on the Chiff River from blocking 02 May 2017, 22:59
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dgr8sandeep
To prevent a newly built dam on the Chiff River from blocking the route of fish migrating to breeding grounds upstream, the dam includes a fish pass, a mechanism designed to allow fish through the dam. Before the construction of the dam and fish pass, several thousand fish a day swam upriver during spawning season. But in the first season after the project's completion, only 300 per day made the journey. Clearly, the fish pass is defective.

Which of the following, if true, most seriously weakens the argument?

(A) Fish that have migrated to the upstream breeding grounds do not return down the Chiff River again.
(B) On other rivers in the region, the construction of dams with fish passes has led to only small decreases in the number of fish migrating upstream.
(C) The construction of the dam stirred up potentially toxic river sediments that were carried downstream.
(D) Populations of migratory fish in the Chiff River have been declining slightly over the last 20 years.
(E) During spawning season, the dam releases sufficient water far migratory fish below the dam to swim upstream.
Show more

Responding to a pm:

Premises:
A fish pass is made on the new dam to allow fish to swim upriver to spawn.
Before the dam, several 1000 fish swam upriver every day.
Now only 300 do per day.

Conclusion: Fish pass is defective.

We have to weaken the argument. So we have to think about why fewer fish may be swimming upstream though the fish pass may not be defective.

(A) Fish that have migrated to the upstream breeding grounds do not return down the Chiff River again.
Irrelevant. What happens to them after the migrate, we don't care. We want to know why they are not migrating.

(B) On other rivers in the region, the construction of dams with fish passes has led to only small decreases in the number of fish migrating upstream.
This strengthens that our fish pass is defective! At other places, the decrease is less. We are seeing a huge decrease.

(C) The construction of the dam stirred up potentially toxic river sediments that were carried downstream.
This provides us a reason why fewer fish are swimming upriver. Perhaps because there are fewer fish now. The construction introduced toxic substances in the river which should have killed many fish. Hence, the fish pass may not be faulty. The problem may be that we have fewer fish now.
Correct.

(D) Populations of migratory fish in the Chiff River have been declining slightly over the last 20 years.
The population has been declining slightly over many years. It doesn't explain the huge decrease in the number of fish swimming upriver.

(E) During spawning season, the dam releases sufficient water far migratory fish below the dam to swim upstream.
The dam is aiding the fish by providing enough water for them to swim upstream. So the fewer fish swimming upstream would not be expected. Hence defective fish pass could actually be the reason.

Answer (C)
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To prevent a newly built dam on the Chiff River from blocking 20 Aug 2016, 07:55
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dgr8sandeep
To prevent a newly built dam on the Chiff River from blocking the route of fish migrating to breeding grounds upstream, the dam includes a fish pass, a mechanism designed to allow fish through the dam. Before the construction of the dam and fish pass, several thousand fish a day swam upriver during spawning season. But in the first season after the project's completion, only 300 per day made the journey. Clearly, the fish pass is defective.

Which of the following, if true, most seriously weakens the argument?

(A) Fish that have migrated to the upstream breeding grounds do not return down the Chiff River again.

(B) On other rivers in the region, the construction of dams with fish passes has led to only small decreases in the number of fish migrating upstream.

(C) The construction of the dam stirred up potentially toxic river sediments that were carried downstream.

(D) Populations of migratory fish in the Chiff River have been declining slightly over the last 20 years.

(E) During spawning season, the dam releases sufficient water for migratory fish below the dam to swim upstream.­


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Conclusion : The fish pass is defective. (X)

Premise : Fishes migrating through the pass has reduced. (Y)

We have X causes Y.

Lets break it. :-D

Option (A) Fish that have migrated to the upstream breeding grounds do not return down the Chiff River again. : Is this a change after the pass or was it happening before also? If before, then it implies it has nothing to do with the conclusion and if after, it implies conclusion is strengthened. Hence , Incorrect.

Option (C) : The construction of the dam stirred up potentially toxic river sediments that were carried downstream. -- Alternate cause for the decrease in number. Hence, weakens the conclusion.

Option (D) : Populations of migratory fish in the Chiff River have been declining slightly over the last 20 years. -- Irrelevant. If it is happening then why did the journey actually reduced after the construction itself. It should have followed the same trend.

Note : We need to find something that explains what happens after construction that made the journey reduction.

Hence, C is the best option with an alternate cause.­
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To prevent a newly built dam on the Chiff River from blocking 03 Sep 2015, 08:22
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Hi drashi,
A weakener just needs to create a doubt in the conclusion. Toxic water may not have killed the fish. But there's a possibility that it could have. In that case, the option weakens the given conclusion.

The author's conclusion is that the fish pass is defective. The basis for this conclusion is that in the first season after the project's completion, only 300 per day made the journey whereas before the construction of the dam and fish pass, several thousand fish a day swam upriver during spawning season.

Possible weakeners for the conclusion:

300 per day doesn't necessarily mean that the fish pass is defective. It could mean:

1) The fish population has gone down.
2) Fish have found a different place other than upstream to go to.

(A) Fish that have migrated to the upstream breeding grounds do not return down the Chiff River again.
-> This is outside the scope of the conclusion. We arent interested in fish that have already migrated upstream.

(B) On other rivers in the region, the construction of dams with fish passes has led to only small decreases in the number of fish migrating upstream.
-> We arent interested in what's happening on other rivers in the region.

(C) The construction of the dam stirred up potentially toxic river sediments that were carried downstream.
-> This is a possible weakener. Toxic river sediments may have brought down the fish population. This is also in accordance with our prethinking.

(D) Populations of migratory fish in the Chiff River have been declining slightly over the last 20 years.
-> This is rather a strengthener. Slight decrease in pouplation doesnt help us to rule out the decrease in population aspect. According to this statement there was no reason why the fish didnt go upstream. Fish pass could thus have been the reason.

(E) During spawning season, the dam releases sufficient water far migratory fish below the dam to swim upstream.
-> This again rules out the possibility that sufficient water wasnt available to go upstream. This is a strenghthner.

Hence, C is the answer.

Hope this helps.
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To prevent a newly built dam on the Chiff River from blocking 04 Feb 2017, 20:31
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Not a very convincing question & answer, I'd say. We are not supposed to bring in our understanding of how the world works, when we solve these questions. How do we know that potentially toxic sediments are harmful for the fish?
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To prevent a newly built dam on the Chiff River from blocking 03 May 2017, 01:08
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Is it correct to assume that the number of fish is reduced/fish died due to potentially toxic sediments? Nothing is mentioned in this argument about this.
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To prevent a newly built dam on the Chiff River from blocking 03 May 2017, 04:32
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Mahmud6
Is it correct to assume that the number of fish is reduced/fish died due to potentially toxic sediments? Nothing is mentioned in this argument about this.
Show more

Yes, it is not mentioned specifically but it alludes to a reason other than a faulty fish pass. That weakens our conclusion. Note that we don't have to establish that the conclusion cannot hold. We just have to make one doubt it, just weaken it.
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To prevent a newly built dam on the Chiff River from blocking 26 Aug 2017, 06:42
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Hi mikemcgarry

I am afraid I need you help

To prevent a newly built dam on the Chiff River from blocking the route of fish migrating to breeding grounds upstream, the dam includes a fish pass, a mechanism designed to allow fish through the dam. Before the construction of the dam and fish pass, several thousand fish a day swam upriver during spawning season. But in the first season after the project's completion, only 300 per day made the journey. Clearly, the fish pass is defective.

Which of the following, if true, most seriously weakens the argument?

(A) Fish that have migrated to the upstream breeding grounds do not return down the Chiff River again.
(B) On other rivers in the region, the construction of dams with fish passes has led to only small decreases in the number of fish migrating upstream.
(C) The construction of the dam stirred up potentially toxic river sediments that were carried downstream.
(D) Populations of migratory fish in the Chiff River have been declining slightly over the last 20 years.
(E) During spawning season, the dam releases sufficient water far migratory fish below the dam to swim upstream.

I picked up E, because,
in the prompt, it states thousands fish swam a per day during spawning season, but in the first season only 300 per day swam, the decreased number is between different seasons.
While choice E states that dam release sufficient water in spawning season, hmmm, E points out a reason, sufficient water from dam, that thousands fish swam, I can get the reason that no sufficient water in the first season after building dam, leading few fish swam,
in other words, it eliminates the factor -- fish pass defective,

Please point out my fault.

Thanks in advance

Have a nice day

>_~
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To prevent a newly built dam on the Chiff River from blocking 10 Sep 2017, 14:19
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VeritasPrepKarishma
dgr8sandeep
(A) Fish that have migrated to the upstream breeding grounds do not return down the Chiff River again.
Irrelevant. What happens to them after the migrate, we don't care. We want to know why they are not migrating.

(B) On other rivers in the region, the construction of dams with fish passes has led to only small decreases in the number of fish migrating upstream.
This strengthens that our fish pass is defective! At other places, the decrease is less. We are seeing a huge decrease.

(C) The construction of the dam stirred up potentially toxic river sediments that were carried downstream.
This provides us a reason why fewer fish are swimming upriver. Perhaps because there are fewer fish now. The construction introduced toxic substances in the river which should have killed many fish. Hence, the fish pass may not be faulty. The problem may be that we have fewer fish now.
Correct.

(D) Populations of migratory fish in the Chiff River have been declining slightly over the last 20 years.
The population has been declining slightly over many years. It doesn't explain the huge decrease in the number of fish swimming upriver.

(E) During spawning season, the dam releases sufficient water far migratory fish below the dam to swim upstream.
The dam is aiding the fish by providing enough water for them to swim upstream. So the fewer fish swimming upstream would not be expected. Hence defective fish pass could actually be the reason.

Answer (C)
Show more

Thank you for your illustrations. I chose A because, when it says these fish are not returning, this offers an explanation that the population of the downstream only decreases, thus making a possibility why only 300 fish made it through the gate(300 fish may be a good level after this population decrease). Is there anything not appropriate with this logic?
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To prevent a newly built dam on the Chiff River from blocking 10 Sep 2017, 21:50
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VeritasPrepKarishma
dgr8sandeep
(A) Fish that have migrated to the upstream breeding grounds do not return down the Chiff River again.
Irrelevant. What happens to them after the migrate, we don't care. We want to know why they are not migrating.

(B) On other rivers in the region, the construction of dams with fish passes has led to only small decreases in the number of fish migrating upstream.
This strengthens that our fish pass is defective! At other places, the decrease is less. We are seeing a huge decrease.

(C) The construction of the dam stirred up potentially toxic river sediments that were carried downstream.
This provides us a reason why fewer fish are swimming upriver. Perhaps because there are fewer fish now. The construction introduced toxic substances in the river which should have killed many fish. Hence, the fish pass may not be faulty. The problem may be that we have fewer fish now.
Correct.

(D) Populations of migratory fish in the Chiff River have been declining slightly over the last 20 years.
The population has been declining slightly over many years. It doesn't explain the huge decrease in the number of fish swimming upriver.

(E) During spawning season, the dam releases sufficient water far migratory fish below the dam to swim upstream.
The dam is aiding the fish by providing enough water for them to swim upstream. So the fewer fish swimming upstream would not be expected. Hence defective fish pass could actually be the reason.

Answer (C)

Thank you for your illustrations. I chose A because, when it says these fish are not returning, this offers an explanation that the population of the downstream only decreases, thus making a possibility why only 300 fish made it through the gate(300 fish may be a good level after this population decrease). Is there anything not appropriate with this logic?
Show more

The argument tells us that there is a season in which fish migrate upstream. In every migration season, 1000 fish were migrating upstream every day. In the first season after the construction of the dam, only 300 fish migrated every day.

Option (A) uses simple present to tell us a fact: Once the fish migrate, they don't come back down.
This was true in every season before the dam was made. Still 1000 fish were migrating every season. Only this season 300 fish are migrating. So option (A) doesn't explain why the fish pass may be ok.
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To prevent a newly built dam on the Chiff River from blocking 28 Mar 2018, 19:57
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Hi mikemcgarry

I am afraid I need you help

To prevent a newly built dam on the Chiff River from blocking the route of fish migrating to breeding grounds upstream, the dam includes a fish pass, a mechanism designed to allow fish through the dam. Before the construction of the dam and fish pass, several thousand fish a day swam upriver during spawning season. But in the first season after the project's completion, only 300 per day made the journey. Clearly, the fish pass is defective.

Which of the following, if true, most seriously weakens the argument?

(A) Fish that have migrated to the upstream breeding grounds do not return down the Chiff River again.
(B) On other rivers in the region, the construction of dams with fish passes has led to only small decreases in the number of fish migrating upstream.
(C) The construction of the dam stirred up potentially toxic river sediments that were carried downstream.
(D) Populations of migratory fish in the Chiff River have been declining slightly over the last 20 years.
(E) During spawning season, the dam releases sufficient water far migratory fish below the dam to swim upstream.

I picked up E, because,
in the prompt, it states thousands fish swam a per day during spawning season, but in the first season only 300 per day swam, the decreased number is between different seasons.
While choice E states that dam release sufficient water in spawning season, hmmm, E points out a reason, sufficient water from dam, that thousands fish swam, I can get the reason that no sufficient water in the first season after building dam, leading few fish swam,
in other words, it eliminates the factor -- fish pass defective,

Please point out my fault.

Thanks in advance

Have a nice day

>_~
Show more


(E) certainly eliminates a problem with the dam, not with the fish pass. In other words, if the dam did NOT release sufficient water, then we would see a decrease in fish migrating upstream EVEN IF the fish pass is totally fine. So (E) eliminates an alternative explanation (problem with the dam itself). If anything, this supports the notion that there might be a problem with the fish pass.

(C), on the other hand, gives a reason why the fish population would have been reduced. This population decrease could explain the decrease in the number that swam upstream. By providing an alternate explanation, (C) weakens the argument.

I hope that helps!
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To prevent a newly built dam on the Chiff River from blocking 06 Oct 2018, 01:18
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VeritasKarishma, GMATNinja

I understand this is old OG question and much discussed already. I have opted for choice C but I have question on option A and C. In option A , it says migrated fishes are not returned, this does give a reason why upstream fish count is reduced. Option C says toxic stuff but it does not mean always that it has killed many fishes reducing the fish count in general. In presence of these 2 observations I think option A make more sense.

Appreciate your views and response. Thanks - N Avinash.
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To prevent a newly built dam on the Chiff River from blocking 20 Oct 2018, 18:17
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VeritasKarishma, GMATNinja

I understand this is old OG question and much discussed already. I have opted for choice C but I have question on option A and C. In option A , it says migrated fishes are not returned, this does give a reason why upstream fish count is reduced. Option C says toxic stuff but it does not mean always that it has killed many fishes reducing the fish count in general. In presence of these 2 observations I think option A make more sense.

Appreciate your views and response. Thanks - N Avinash.
Show more
The conclusion of this argument (the fish pass is defective) is concerned with fish that are migrating to breeding grounds upstream.

Quote:
(A) Fish that have migrated to the upstream breeding grounds do not return down the Chiff River again.
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Choice (A) has nothing to do with fish that are migrating to breeding grounds upstream.

This choice describes fish that have already migrated to the upstream breeding grounds. These fish are irrelevant to the argument we are asked to evaluate and weaken. That's why we eliminate (A). I hope this clears the waters!
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To prevent a newly built dam on the Chiff River from blocking 21 Oct 2018, 14:12
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I guess my brain is demented. Can anyone help this demented brain understand why option A to be wrong?
I have read the experts reply. I am still not able to digest why A is wrong? Fish that have migrated upstream breeding grounds don't return the chiff river again.
If there were several thousand fishes and if they had already migrated(they would not want to go downstream for some reason). Only a few would have been left. These few would try to migrate using the fish pass thus attributing to the fewer fish that use the fish pass.
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To prevent a newly built dam on the Chiff River from blocking 23 Oct 2018, 02:34
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I guess my brain is demented. Can anyone help this demented brain understand why option A to be wrong?
I have read the experts reply. I am still not able to digest why A is wrong? Fish that have migrated upstream breeding grounds don't return the chiff river again.
If there were several thousand fishes and if they had already migrated(they would not want to go downstream for some reason). Only a few would have been left. These few would try to migrate using the fish pass thus attributing to the fewer fish that use the fish pass.
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Every year, during spawning season, several 1000s of fish swim upriver to breeding grounds.
This happens during every spawning season. If the fish that have migrated do not come back, they do not come back every season. But still 1000s of fish swim upriver every day in every spawning season.

But in this spawning season (the first one after the dam got constructed), only 300 fish are swimming upriver. So either there is a problem with the fish pass or there are fewer fish or something else. Certainly, the new dam has had some impact this season.
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To prevent a newly built dam on the Chiff River from blocking 13 Dec 2018, 05:38
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I used Pre-thinking technique, Now the issue is that the first thought I had was that there might have been a population decline because once the migration was done the fish never came back to their originating position. In such a scenario why option A is wrong?
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To prevent a newly built dam on the Chiff River from blocking 13 Dec 2018, 05:46
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I used Pre-thinking technique, Now the issue is that the first thought I had was that there might have been a population decline because once the migration was done the fish never came back to their originating position. In such a scenario why option A is wrong?
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Note the tone of the argument:

Before the construction of the dam and fish pass, several thousand fish a day swam upriver during spawning season. But in the first season after the project's completion, only 300 per day made the journey.

During spawning season, several 1000 fish used to swim upriver as a habitual thing. It used to happen in every spawning season. So this was not causing decrease in population. The change happened only after the project's completion.
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rish2708
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To prevent a newly built dam on the Chiff River from blocking 26 Dec 2018, 07:00
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Thanks GMATNinja and VeritasKarishma for the wonderful explanations. It helped me rule out E to an extent.

I have made up a pictorial visualisation for this question, to understand it properly.
If possible, could you please assess the validity of the understanding as attached :)

My small query is:

There are two scenarios:

Before the construction of the dam and fish pass, several thousand fish a day swam upriver during spawning season

But in the first season after the project’s completion, only 300 per day made the journey.

Here we have FIRST SEASON and SPAWNING SEASON. Are they both same? How?
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File comment: Pictorial representation of the explanation.
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KarishmaB
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To prevent a newly built dam on the Chiff River from blocking 01 Jan 2019, 06:47
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rish2708
Thanks GMATNinja and VeritasKarishma for the wonderful explanations. It helped me rule out E to an extent.

I have made up a pictorial visualisation for this question, to understand it properly.
If possible, could you please assess the validity of the understanding as attached :)

My small query is:

There are two scenarios:

Before the construction of the dam and fish pass, several thousand fish a day swam upriver during spawning season

But in the first season after the project’s completion, only 300 per day made the journey.

Here we have FIRST SEASON and SPAWNING SEASON. Are they both same? How?
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Yes, we are talking about only one season and that is the spawning season. The first season after the dam opens means first spawning season.
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To prevent a newly built dam on the Chiff River from blocking 02 Nov 2019, 23:32
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Hi Experts,

I have doubt on the meaning " have been declining slightly over the last 20 years"
I understand that the interpretation of the phrase would be:
" Populations have been declining slightly every year for the last 20 years: that is, let's say the declining rate were 1% each year -- or it would be 2% ,3%,4% which are quite subjective but can impact the argument-- , the total declining rate would amount to about 20%,
which is quite large.

So, I think D could be the answer.

Please explain.

Thank you.
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