Question Pack 1 | The Earth's rivers constantly carry dissol

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The Earth's rivers constantly carry dissolved salts into its oceans. Clearly, therefore, by taking the resulting increase in salt levels in the oceans over the past hundred years and then determining how many centuries of such increases it would have taken the oceans to reach current salt levels from a hypothetical initial salt-free state, the maximum age of the Earth's oceans can be accurately estimated.

Which of the following is an assumption on which the argument depends?

A. The quantities of dissolved salts deposited by rivers in the Earth's oceans have not been unusually large during the past hundred years.
B. At any given time, all the Earth's rivers have about the same salt levels.
C. There are salts that leach into the Earth's oceans directly from the ocean floor.
D. There is no method superior to that based on salt levels for estimating the maximum age of the Earth's oceans.
E. None of the salts carried into the Earth's oceans by rivers are used up by biological activity in the oceans
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by richachampion » Tue Apr 11, 2017 12:32 pm
OA: A

Although, I was able to solve this, but still I am inviting experts to provide their perspective.
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by richachampion » Tue Apr 11, 2017 12:37 pm
I believe that Option C and Option E are very similar to each other. Therefore they both should be discarded. Thanks!
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by DavidG@VeritasPrep » Wed Apr 12, 2017 11:31 am
richachampion wrote:The Earth's rivers constantly carry dissolved salts into its oceans. Clearly, therefore, by taking the resulting increase in salt levels in the oceans over the past hundred years and then determining how many centuries of such increases it would have taken the oceans to reach current salt levels from a hypothetical initial salt-free state, the maximum age of the Earth's oceans can be accurately estimated.

Which of the following is an assumption on which the argument depends?

A. The quantities of dissolved salts deposited by rivers in the Earth's oceans have not been unusually large during the past hundred years.
B. At any given time, all the Earth's rivers have about the same salt levels.
C. There are salts that leach into the Earth's oceans directly from the ocean floor.
D. There is no method superior to that based on salt levels for estimating the maximum age of the Earth's oceans.
E. None of the salts carried into the Earth's oceans by rivers are used up by biological activity in the oceans
Take a simplified example of the phenomenon described. Imagine that you have a body of water with 10 ounces of salt in in. Last year, the salt level increased by 2 ounces. Therefore, you conclude that the body of water, which is taking on salt at 2 ounces per year, is 5 years old. A big assumption here is that the salt level has been increasing at a constant rate. If last year the salt increased by 2 ounces, but previously, it had been increasing by, say .01 ounces, you're going to radically underestimate the age of the body of water if you assume that it's always taken on 2 ounces per year.

A negated: The quantities of dissolved salts deposited by rivers in the Earth's oceans have been unusually large during the past hundred years.

If the amount of salt deposited over the past hundred years is unusually high, then we're not going to be able to estimate the age of the oceans accurately using the past century as a benchmark for salt increases during previous centuries. Because the negation destroys the argument, A must be correct.
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by DavidG@VeritasPrep » Wed Apr 12, 2017 12:37 pm
richachampion wrote:I believe that Option C and Option E are very similar to each other. Therefore they both should be discarded. Thanks!
They're not exactly the same.

C negated: There are not salts that leach into the Earth's oceans directly from the ocean floor.
It doesn't matter if salt comes from sources other than rivers. What we care about is the overall rate at which the salt is increasing. Because the negation of C is irrelevant, it's wrong.

E negated: Some of the salts carried into the Earth's oceans by rivers are used up by biological activity in the oceans.
This doesn't undermine the conclusion. It's possible that "some" refers to a negligible amount of salt. So long as the salt is increasing at the same rate over many centuries, we'd still be able to estimate the ocean's age. E is out.
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by richachampion » Sun May 07, 2017 10:58 pm
DavidG@VeritasPrep wrote:They're not exactly the same.
Actually, sir, they are the one and the same in one respect. I am not a native speaker so my expression power is not that good, but still, I will try to explain what I mean.

C. There are salts that leach into the Earth's oceans directly from the ocean floor.
E. None of the salts carried into the Earth's oceans by rivers are used up by biological activity in the oceans

We are relying on the net increase/century.
Salts ± Other Sources = resulting increase
Notice I have used both "+" and "-" signs because other sources may add or subtract to the salt added by rivers, but remember that we are concerned about net increase.
This is not an assumption you can test it by negating this option, and it will never break the conclusion.

I am giving you an analogous example suppose there is a boy by a name of Gajat Rugnani and his mother gives him money to deposit

+VE Example: Gajat Rugnani is a nice boy and he works hard and earn 10$ every month andd that to the 100$ given by his mother and he deposit every month 110$, so when he has 11000$ and we know that he deposits 110$/month we can safely calculate w/o any error that he has been depositing since 100 months

-VE Example: Gajat Rugnani is a dishonest and naughty boy 100$ every month and he is dishonest and he spends 10$, but deposit only 90$ in his Piggy Bank. So based on a constant depoist every month if we have 9000$ in his Piggy bank we can safley calculate that he has deposited for 100 months despite 10$ was reduced every month, but still it didnt cause erropr in calculating how many months since he is depositing.

Now irrespective of whether Gajat Rugnani is hardworking or dishonest or none, and thus irrespective of whether
Gajat Rugnani deposits 90$, 100$, or 110$ based on the resultant we can safley calculate the total # of months of his deposit.

Thus in our main option too resultant deposit matters and as such it is not a unstated assumption because assumption is something w/o which conclusion/argument can't stand.

Let me add little , as per reasoning author states that one can extrapolate last 100 years data till end to calculate the ocean's age. so assumption should be that extrapolation(linear) is possible and no randomness between centuries.

A addresses this well, where as in E ,apart from extreme word, biological activity can also be extrapolated to all previous centuries . Since its doesn't convey randomness between centuries.

Hope it makes sense.
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by richachampion » Sun May 07, 2017 11:23 pm
DavidG@VeritasPrep sir, Did you get my point. The two options are literally not same not rephrased, but logically they are same with same effect and 2 answers can't be true and thus can be discarded. I have seen this pattern in many GMAC problems.
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by DavidG@VeritasPrep » Mon May 08, 2017 8:13 am
richachampion wrote:DavidG@VeritasPrep sir, Did you get my point. The two options are literally not same not rephrased, but logically they are same with same effect and 2 answers can't be true and thus can be discarded. I have seen this pattern in many GMAC problems.
Well, if your point is that the answer choices are similar in the sense that neither qualifies as a necessary assumption because neither addresses the issue of the rate of change over time, that's fair enough. Thinking about it more broadly, I'd say the reasonable lessons to draw from this question are as follows:

1) If an answer choice is irrelevant to the conclusion, it's wrong
2) If two answer choices convey the same information, they must both be wrong

We're on the same page here. But I'll add one more.

3) If two answer choices feel similar, but there could be some reasonable disagreement about whether they're identical, consider each option's impact on the conclusion separately just to be safe.
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