The Earth's rivers constantly carry dissolved salts into its oceans.

We can eliminate B, C and D after reading.
The argument is talking about: We can guess the maximum Earth's age when we calculate dissolved salts from rivers to oceans with the premise is at the first time, the salt level of oceans is zero.

B: Irrelevant because we do not discuss about salt level in rivers. It does not lead us to the conclusion.
C: Irrelevant, too.
D: Although it seems to be a right answer because it talks about a method. However, it is too general, and not assume anything.
E: The first time, I think E is the best answer. However, it is a mistake. First, it is too specific. If none of the salts were used by biological activity, so, how about other activities? (such as physical activity, chemical activity...)? Second, it does not lead us to a conclusion. We cannot conclude that the method is accurate because of the premise E.
A: It supports the premise:
Premise:
- At the first time, the salt level of oceans is zero.
- We calculate the salt level of oceans in the past hundred years, and we can know the Earth's age.
- In a process, the quantities of dissolved salts deposited by rivers in the Earth’s oceans have not been unusually large during the past hundred years.
- Thus, we can convince that the result is believable.

I hope that you guys can accept my explanation.

Consider A,
A. The quantities of dissolved salts deposited by rivers in the Earth’s oceans have not been unusually large during the past hundred years.

Negate it ->

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

Clearly then our experiment to determine age of the oceans will fail.
So, this is an assumption the argument relies upon.

IMO - A

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

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 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.

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.

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
Arun

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

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

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

A will be the correct answer .

E states that the salt is not being used by any biological activity . But , if we are to measure the age of the oceans this will not hamper that measurement as the quantity of salt, if being used by any biological activity would have been constant since the beginning . So , that constant will not affect the measurement .

A assumes that the quantity of salts deposited in recent hundred years is larger than other hundreds of years , thus the experiment will give an inconsistent answer .

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 ocean to reach current salt levels from a hypothetical initial salt-free state, the maximum age of the Earth's oceans can be accurately estimate.

Which of the following is the 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.

GMATNinja & GMATNinjaTwo (A), (B) and (E) are pretty close to each other. How to eliminate the incorrect answer choices?

As for choice (B), we don't care about the relative salt levels in the individual rivers, only about the cumulative amount of salt carried into the oceans. It doesn't matter if some rivers carry more salt than others.

As for choice (E), "taking the resulting increase in salt levels in the oceans over the past hundred years" will tell us how much the salt levels increased DESPITE loss of salt due to biological activity (or any other factor that would reduce salt levels in the ocean). If the impact of these factors is small, these factors will not significantly affect the calculations. In other words, it is okay if A SMALL AMOUNT of salt is used up by biological activity in the oceans.

This thread contains several other posts explaining (B) and (E), so please review these before posting additional questions.

Assume we know in hundred years salt level increases by 10 units and 1 unit is used by biological activity. So effectively salt level increases by 9 units every hundred years. So if we know increase in the salt level in the past hundred years ,we can find out how long it took to reach the current level from salt free level even if some salt is used by biological activity. The biological activity would have been there all along. E need not be assumed

Hi GMATNinja

i think your reason for eliminating E is saying -- this sentence "taking the resulting increase in salt levels in the oceans over the past hundred years" could account / could already incorporate for loss of salt due to biological activity (or any other factor that would reduce sea salt levels in the ocean)

Question :

as a test taker -- how would one figure out if this sentence ["taking the resulting increase in salt levels in the oceans over the past hundred years"] could / could not account for this ?

Its quite possible that the sentence ["taking the resulting increase in salt levels in the oceans over the past hundred years"] has not thought about this possibility [ i.e. the loss of salt due to biological activity (or any other factor that would reduce sea salt levels in the ocean)

The question asks us to determine which answer choice is an assumption "on which the argument depends." In other words, the correct answer choice MUST be true in order for the argument to hold.

So, it is enough to show that (E) does not HAVE to be true -- the argument COULD already account for salt being used up by biological activity, so (E) is not an absolutely necessary assumption.

(A), on the other hand, MUST be true. The argument relies on salt levels for the past hundred years providing an accurate estimate of the maximum age of the world's oceans. If the quantities of salt deposited by rivers for the past hundred years have been unusually high, then the inference about the age of the oceans cannot be made. So, the author MUST assume that the quantities of salt deposited by rivers have not been unusually high.

I hope that helps!

(A) The quantities of dissolved salts deposited by rivers in the Earth’s oceans have not been unusually large during the past hundred years.
Yes it should be a constant value not having any dramatic shifts

(B) At any given time, all the Earth’s rivers have about the same salt levels.
There is a possiblity that each river can have constant variable individually

(C) There are salts that leach into the Earth’s oceans directly from the ocean floor.
This is out of context

(D) There is no method superior to that based on salt levels for estimating the maximum age of the Earth’s oceans.
We are only discussing one method we are not in a position to determine the same

(E) None of the salts carried into the Earth’s oceans by rivers are used up by biological activity in the oceans
Thhis also is out of context
Hence IMO A