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The Earth's rivers constantly carry dissolved salts into its [#permalink]
 20 May 2005, 11:53
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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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I will go with B
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Re: CR - earth's rivers [#permalink]
20 May 2005, 23:18
B.At any given time, all the Earth's rivers have about the same salt levels.
Let me understand B as "every year all the rivers deposited the same level of salt"
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E.
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Director
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(B) too.
I understand the stimulus as saying that a steady proportion of salt in oceans can be used to determine the age of the oceans. that presumes a steady proportion of salt in the rivers that dump salty water (salt) to the ocean.
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B.
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Why cant it be E?
If the salts are being used up due to other factors,then obviously the conclusion is wrong.This is the biggest assumption made.
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Between B and D, i choose D. 
Best.
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IMO its E)...because when the salt is used by activities in the oceans, the data of the concentration of salt in the oceans is not accurate and the maximum age cannot be determined.
the negation of B) doesnt attack the conclusion. the argument says "...by taking the resulting increase in salt levels in the oceans...". it implies that the oceans have different levels or at least it doesnt imply that they have the same levels.
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I think the answer is A. If the salt levels in the last 100 years is unusually large, then the age of earth determined using this result will obviously be far from accurate.
As per E, if the biological activity is constant over the last million/trillion years, it would still be possible to determine the age of the earth's oceans accurately.
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christoph wrote: IMO its E)...because when the salt is used by activities in the oceans, the data of the concentration of salt in the oceans is not accurate and the maximum age cannot be determined.
the negation of B) doesnt attack the conclusion. the argument says "...by taking the resulting increase in salt levels in the oceans...". it implies that the oceans have different levels or at least it doesnt imply that they have the same levels.
I agree with above reasoning
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Re: CR - earth's rivers [#permalink]
23 May 2005, 21:26
The method: measure total salt in the late hundred years (S). Assuming S is constant over years (S0=S), we can then divide the total salt (T) by S to get maximum age of earth's ocean.
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 S is greater than S0 then the result T/S<T/S0. The result would not be the maximum age of earth's ocean.
B.At any given time, all the Earth's rivers have about the same salt levels.
You don't have to assume all river has the same salt level, as long as total salt per hundred years are constant.
C.There are salts that leach into the Earth's oceans directly from the ocean floor.
Wonder if it should be "are" or "aren't". For we definitely don't need to assume something that would make the whole thing more complicated. 
D.There is no method superior to that based on salt levels for estimating the maximum age of the Earth's oceans.
Irrelevant
E.None of the salts carried into the Earth's oceans by rivers are used up by biological activity in the oceans
If there are biological activity that uses salt, T0>T, T/S<T0/S. T/S would not be the maximum age.
I can't decided bt A and E.
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E connects the premise to the conclusion........
B is a CONSTANT value of salinity from the rivers into the ocean.
The salinity of the ocean may rise at a constant pace but the aquatic/marine life may take their chunk.........E exactly tells us about this
I would pick E
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E it is.
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Tough choice between A and E, but I lean towards E because in A, the unusually large deposit over the past 100 years or so may skew the result, but in the grand scheme of total ocean age (potentially milions of years), this aberratioin will be so tiny it won't really matter. But in E, since marine creatures have existed for much much longer than the past 100 years, the total cumulative salt consumption (if they do consume it) would be big enough over the millions of years to seriously undermine the age calcution.
OA please ?
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My choice : A
In "E" even if salt is being consumed by the ocean for biological activities, we still don't know whether this consumption has been uniform in history. If it is, the argument still holds. If it is not uniform, argument falls apart. Either way we don't know abt this and argument doesn't rely on this to reach its conclusion.
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A
I first read B as the salinity of a river being constant over time. This could distort the ratio. But on second readig, it is clear that the author is referring the salinity being constant for all the rivers. So B wud be out.
I wud rule out E becos even if there was a biological activity it wud apply for all the years in the life of an ocean. Hence the max age wud kind of be constant.
If there was an unusual increase in salinity in the last 100 years, it wud distort the max age.
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I guess it shud be E....
Others are irrelevant....
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I think it's A. becaise even some salt have been used, we still can get the MAXIMUM age of ocean.
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I am confused..... I shall change over from E to A 'coz-
even if the salt is consumed for some reason (as per E) it should not matter as far the first statement holds good-
"Earth's rivers constantly carry dissolved salts into its oceans".- "constantly" is a key word here...
Also the arguments states 'maximum age of the Earth's oceans can be accurately estimated"..."accurately" could be the catch....
If A is not true then we cannot measure the age "accurately" & hence it is an assumption...
So I believe A makes sense..!!!!!!!!
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