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Earth's Rivers

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Earth's Rivers

by AnjaliOberoi » Wed May 28, 2014 11:31 pm
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.

OA is A. Can someone help me in understanding that why A is correct and E is wrong.
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by aditya8062 » Thu May 29, 2014 12:12 am
E says:None of the salts carried into the Earth's oceans by rivers are used up by biological activity in the oceans.

even if u say that some of the salt is used up by biological activity then it will not affect our analysis as we are basing our calculation on the increase of salt content .think it otherwise if some biological activity is indeed consuming some amount of salt then that consumption will be taken care of in the "increase of salt content"

hence E is not indispensable for my argument and hence it is not an assumption
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by AnjaliOberoi » Thu May 29, 2014 12:14 am
can you help me with the explanation for A as well.

Thanks
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by theCodeToGMAT » Thu May 29, 2014 12:43 am
Salt to Ocean --> process can be made to calculate Earth's Ocean Age.

Conclusion: " the maximum age of Earth's oceans can be calculated accurately"

Using Negation Technique
We need to show that we cannot accurately calculate

Now, lets negate the choices:
{A} Quantities have been unusually large during part hundred year ---> this tells us that if we rely on the data of last 100 years, then we may end up with inaccurate data. KEEP
{B} Earth's rivers do not have the same salt levels. --> doesn't show the inaccuracy IGNORE
{C} There are no salts that leach --> Strengthens the Conclusion IGNORE
{D} There is method --> Doesn't tell us anything IGNORE
{E} All salts are used by biological Activity in ocean. --> So, doesn't affect the accuracy. pattern can be generated IGNORE

[spoiler]{A} [/spoiler]
R A H U L
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by aditya8062 » Thu May 29, 2014 12:45 am
the argument is posing: that by measuring the increase in the amount of salt over the years we can measure the age of the ocean ------>here the author is making the assumption that this increase is constant over the years, a fact that might not be true .

now look at A
A says: The quantities of dissolved salts deposited by rivers in the Earth's oceans have not been
unusually large during the past hundred years.

in other words A is exactly what i had thought of as assumption

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

now if the salts deposits were unusually large during past hundred years then my calculation will go wrong .and i will end up with wrong age of oceans !!
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by GMATGuruNY » Thu May 29, 2014 6:12 am
AnjaliOberoi 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.
This is essentially a PLANNING argument.
Plan: Examine salt deposits over the past 100 years.
Goal: Determine the maximum age of the Earth's oceans.

Apply the NEGATION TEST.
The correct answer choice -- the assumption -- is WHAT MUST BE TRUE for the plan to achieve its goal.
Thus, when the correct answer choice is negated, the plan will fall apart.

Answer choice 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 salt deposits over the past hundred years have been exceptionally large -- if they are NOT representative of salt deposits since the oceans' INITIAL SALT-FREE STATE -- then we cannot use these deposits to determine the maximum age of the Earth's oceans.
Since the negation of A invalidates the plan, A is the correct assumption: WHAT MUST BE TRUE for the plan to work.

The correct answer is A.

The negation of none is SOME.
Answer choice E, negated:
Some of the salts carried into the Earth's oceans by rivers are used up by biological activity in the oceans.
Here, some of the salts could represent a TINY AMOUNT, with the result that the plan is unaffected.
Also, there probably has been biological activity since the oceans' initial salt-free state, so the negation of E does not explain why studying what has happened over the past hundred years is not a valid way to determine the age of the oceans.
Since the negation of E does not invalidate the plan, eliminate E.
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by shobhitk » Sun Jun 01, 2014 4:13 am
I agree with the responses above and even I have marekd the same answer. But I took longer than expected as I got confused between A and D. I usually do get confused in such questions. Here is why. As in D, if there is a better method to calculate the earth's age, doesn't the conclusion of identifying earth's age accurately will get affected? Hence on negation, even D should hurt the conclusion.

I know there is a minor gap in the way I'm looking at things. Can anyone please help clarify?
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by GMATGuruNY » Sun Jun 01, 2014 4:39 am
shobhitk wrote:I agree with the responses above and even I have marekd the same answer. But I took longer than expected as I got confused between A and D. I usually do get confused in such questions. Here is why. As in D, if there is a better method to calculate the earth's age, doesn't the conclusion of identifying earth's age accurately will get affected? Hence on negation, even D should hurt the conclusion.

I know there is a minor gap in the way I'm looking at things. Can anyone please help clarify?
The argument concludes that taking the resulting increase is salt levels is a valid way to estimate the maximum age of the Earth's oceans.
It does NOT conclude that this method is the ONLY way or the BEST way to estimate the age of the oceans.
Even if there is a superior way to measure the age of the oceans, the salt-level method could still be perfectly valid.
Thus, the negation of D does not invalidate the conclusion.
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by BTGmoderatorAT » Fri Sep 01, 2017 8:50 am
I go also for Letter 'A'

Does Global Warming has important role in determining the Earth's age? As far as I know, it's effect makes the ocean less salty.
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