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The Earth s rivers constantly carry dissolved salts into its

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The Earth s rivers constantly carry dissolved salts into its [#permalink]

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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.
[Reveal] Spoiler: OA
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Re: CR Salt content [#permalink]

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between A and E...
one of the best ways for CR assumption questions
negate the assumption and the argument should fall apart.

negate E
Some of the salts carried into the Earth’s oceans by rivers are used up by biological activity in the oceans.
OK.. but if the portion of salt consumed by biological activities has remained constant over the years ... we can still predict the age of the oceans ... the argument doesnt fall apart

negate A
The quantities of dissolved salts deposited by rivers in the Earth’s oceans have been unusually large during the past hundred years.
it means that the rate has not been constant .... we cant predict the age .... agrument falls apart ...

right Option A
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Re: CR Salt content [#permalink]

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priyankur_saha@ml.com wrote:
IMO E.
* The quantities of dissolved salts deposited by rivers in the Earth’s oceans have not been unusually large during the past hundred years.
Deposit could be large but that never hampers the result


If the quantities of dissolved salts deposited by rivers in the Earth’s oceans have been unusually large during the past hundred years , it will surely affect the result.

Consider the salts deposited in the past century is X.

The current salt level is say 5X

so as per the result ocean's age is 5 centuries.

But if X is unusually large and the deposited salt level of previous centuries were say only X/4 per century then the result will have a blunder!
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Re: CR Salt content [#permalink]

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Conclusion: the maximum age of the Earth’s oceans = (Current Salt Level - hypothetical initial salt-free state) / (the oceans over the past hundred years)

Assumption: For every year there is constant salt deposit in the ocean. If not, this argument falls apart.

B
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Re: CR Salt content [#permalink]

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+1 for OA.

Have not been large is same as constant.. i.e. over the last 100 years the the amount of salt dissolving into sea is constant
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Re: CR Salt content [#permalink]

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hibloom wrote:
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.

i was a difficult choice between a and e.
I eliminated E with the foll. reasoning.
If at the beginning of the last century,the ocean contained x kgs of salt.
If in the last 100 years , 100 kgs of ice was dropped into the ocean by the rivers.
and suppose that in the past 100 yrs 5 kgs of salt was used up by the ocean because of its biological activity.
then 95 kgs was dropped in the last century.
So the level becomes x + 95
still we can find out the age of the ocean by taking 95 kgs as the average increase in 1 century.
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Re: CR Salt content [#permalink]

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I initially had a strong urge to choose E after I narrowed down A and E.

However after some time thought option E could be broken but not A. Here is why:

Option E talks about "none of the salt getting used up". By taking the resulting increase in salt levels in the oceans over the past hundred years we could accommodate the the condition wherein some of the salt is being used up.

Hence option E is not the safe assumption to make. Hence A is the answer.
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Re: CR Salt content [#permalink]

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Has to be A as biological activity will happen every year.

So if rivers bring salt S and biological consumption is s then for a year the Salt increase would be S-s.
So even if we look at a long period every year s amount of salt gets consumed by biological consumption.
That way we can calculate the no of years based on X/(S-s) where X is total increase in the Salt content. So biological activity should not be a problem.

Thanks
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Re: CR Salt content [#permalink]

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Has to be A. At no point does the argument make any assumptions about the river's salt content being the same. Since rivers dump into the ocean their salinity can be different. It does make the assumption that at one point the ocean had a salt free base that was likely equal to the rivers, but this is not what B says. So it has to be A.
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Re: CR Salt content [#permalink]

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hibloom 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.


The method is purely based on the observations from the past 100 years, so we need to something which points out the flaw in this method.

(A) The quantities of dissolved salts deposited by rivers in the Earth’s oceans have not been unusually large during the past hundred years. CORRECT. this says the rate of salt deposited in the last 100 years is unusually high. This clearly weakens the argument.
(B) At any given time, all the Earth’s rivers have about the same salt levels. irrelevant
(C) There are salts that leach into the Earth’s oceans directly from the ocean floor. if salt leaches into the oceans directly, it would have done so in the past 100 years as well. so it doesnt point to a flaw in the proposed method.
(D) There is no method superior to that based on salt levels for estimating the maximum age of the Earth’s oceans.
Ignore it
(E) None of the salts carried into the Earth’s oceans by rivers are used up by biological activity in the oceans.
if oceans use salt for biological activities. it would have done so in the past 100 years as well. so it doesnt point to a flaw in the proposed method
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Re: CR Salt content [#permalink]

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A tough one and after much deliberation I choose option A.

To simplify this lets consider that I can walk at a constant rate of 10 km/hr. On a given day I walk 20 kms. It took me 2 hours to cover the distance. The result (time to cover the distance) is depending on the walking rate and not the distance I covered. If I start walking at a higher rate it will take me less time to cover the distance.

Similarly the age of the ocean = Increase in salt level/rate of deposition per century. The assumption is that the rate of deposition is constant; otherwise the result will be inaccurate.

Option A: The quantities of dissolved salts deposited by rivers in the Earth’s oceans have not been unusually large during the past hundred years. –this means that the rate of deposition is constant and thus we can accurately calculate the age of the ocean.

Using method of negation
Option E – All (polar opposite of none) of the salts carried into the Earth’s oceans by rivers are used up by biological activity in the oceans. – extreme assumption. Moreover it doesn’t weaken the conclusion as we can still calculate the age of the ocean.

Option A - The quantities of dissolved salts deposited by rivers in the Earth’s oceans have been unusually large during the past hundred years. – the rate of deposition is not constant. The conclusion falls apart. We can’t accurately calculate the age of the ocean
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Re: The Earth s rivers constantly carry dissolved salts into its [#permalink]

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Level of salt in the ocean = could be use to predict the age of oceans...
How? determining the number of centuries it took to reach current salt-level from less-salt state...

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.
If salt level in the past was also unusually large as now, then perhaps there is no such thing as increase in salt levels that could help determine the age...

(B) At any given time, all the Earth’s rivers have about the same salt levels.
Rivers can have different salt levels and the conclusion will still hold... This is not assumed...

(C) There are salts that leach into the Earth’s oceans directly from the ocean floor.
If there are salts from the ocean floor and salts from river, the comparison of the levels of salt today and past hundred years will still work as an age determinant... TThe conclusion will still hold...

(D) There is no method superior to that based on salt levels for estimating the maximum age of the Earth’s oceans.
Extreme... There's no claim about the superiority of these method compared to other methods...

(E) None of the salts carried into the Earth’s oceans by rivers are used up by biological activity in the oceans.
Even if salts are used up by the oceans, as long as that consumption rate is the same althroughout.. the method will still work.. This trap is similar to (C) in that it introduces factors other than rivers to contribute to change in salt levels... (e.g. Ocean floor and Biological Process) as long as the salt levels did increase from past to now... then the method will work...

Answer: A
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Re: CR Salt content [#permalink]

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New post 15 Aug 2008, 01:23
hibloom 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?

* The quantities of dissolved salts deposited by rivers in the Earth’s oceans have not been unusually large during the past hundred years.
* At any given time, all the Earth’s rivers have about the same salt levels.
* There are salts that leach into the Earth’s oceans directly from the ocean floor.
* There is no method superior to that based on salt levels for estimating the maximum age of the Earth’s oceans.
* None of the salts carried into the Earth’s oceans by rivers are used up by biological activity in the oceans.


Between A & E.

I am more inclined towards A though, but confusing.
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Re: CR Salt content [#permalink]

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ill say E. Without E, the authors method wont work since he relies on none of the salt being used up .... if it were, the age would be inaccurate
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Re: CR Salt content [#permalink]

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New post 15 Aug 2008, 07:06
IMO E.
* The quantities of dissolved salts deposited by rivers in the Earth’s oceans have not been unusually large during the past hundred years.
Deposit could be large but that never hampers the result
* At any given time, all the Earth’s rivers have about the same salt levels.
OOS
* There are salts that leach into the Earth’s oceans directly from the ocean floor.
OOS
* There is no method superior to that based on salt levels for estimating the maximum age of the Earth’s oceans.
OOS
* None of the salts carried into the Earth’s oceans by rivers are used up by biological activity in the oceans.
If biological process uses the same salt, concentration will definitely fall
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Re: CR Salt content [#permalink]

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New post 15 Aug 2008, 12:33
hey alpha y dont you share your line of reasoning
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Re: CR Salt content [#permalink]

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New post 15 Aug 2008, 19:24
Only B & E makes sense

B : only if the salt content carried by the river is constant, will the calculation of salt content in the ocean be possible.

E : this seems to more directly implied than the previous one.

Hence I would go with E

Whats the OA??
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Re: CR Salt content [#permalink]

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New post 15 Aug 2008, 22:05
alpha_plus_gamma wrote:
priyankur_saha@ml.com wrote:
IMO E.
* The quantities of dissolved salts deposited by rivers in the Earth’s oceans have not been unusually large during the past hundred years.
Deposit could be large but that never hampers the result


If the quantities of dissolved salts deposited by rivers in the Earth’s oceans have been unusually large during the past hundred years , it will surely affect the result.

Consider the salts deposited in the past century is X.

The current salt level is say 5X

so as per the result ocean's age is 5 centuries.

But if X is unusually large and the deposited salt level of previous centuries were say only X/4 per century then the result will have a blunder!

I'm confused between A and E, but I think A is the best. Partly I agree with alpha_plus_gamma, partly I think E is too an extreme assumption
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Re: CR Salt content [#permalink]

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New post 15 Aug 2008, 22:14
OA is A
nice explanation by alpha
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Re: CR Salt content [#permalink]

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New post 05 Oct 2010, 07:10
alpha_plus_gamma wrote:
priyankur_saha@ml.com wrote:
IMO E.
* The quantities of dissolved salts deposited by rivers in the Earth’s oceans have not been unusually large during the past hundred years.
Deposit could be large but that never hampers the result


If the quantities of dissolved salts deposited by rivers in the Earth’s oceans have been unusually large during the past hundred years , it will surely affect the result.

Consider the salts deposited in the past century is X.

The current salt level is say 5X

so as per the result ocean's age is 5 centuries.

But if X is unusually large and the deposited salt level of previous centuries were say only X/4 per century then the result will have a blunder!

the problem says "...from a hypothetical initial salt-free state..."

in every century, there could be an increase and this increase should be roughly constant, then we could calculate the result. if portion of the salt was used in other way, ie. not calculated, then the result cannot be produced.

imo, E.
Re: CR Salt content   [#permalink] 05 Oct 2010, 07:10

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