confused answer for " dissolved salt" why not E?

[elaine.ni.yelin]

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.

[tomslawsky]

I say "A" because if we are estimating billions of years based on 100 years of consecutive data, we must assume the 100 years is not an outlier.

I say "not E" because one can assume that if salt is being used up now, it has been used up since the inception of the ocean, therefore, although being used up now, it has always been used up.

As an extra thought, a good assumption of the argument would be that salt has been always used up at a constant rate.

I could be wrong, what is the answer?

[whuannou]

I would say A.

E does not seem right for the following reason:

To estimate the age of the oceans one will divide "ocean's current salt levels" A by "the resulting increase in salt levels in the oceans over the past hundred years" a.

Whether or not "the salts carried into the Earth’s oceans by rivers are used up by biological activity in the oceans" will not change anything to the reasoning.
If biological activities use some of the salt, both quantities A and a will decrease correspondingly and the calculation should lead to the same result.

What is the answer?

[Ben Ku]

Please cite the source (author) of this problem. We cannot reply unless a source is cited (and, if no source is cited, we will have to delete the post!). Thanks.

[aru]

Hi Ben, This is a GMAT prep question pack 1 question now. Can you please elaborate why E is a superior answer to A.

regards

[tim]

give us a screen shot. Ben and i are both skeptical of this problem, and we want to see that it is a real GMAT prep question and get confirmation of what GMAT thinks the answer is..

[parthian7]

http://postimage.org/image/y5imzr1hh/

[tim]

Okay looks like the problem is legit and the answer is A. Does anyone have a question on this one that has not yet been answered? Arushi, please note that given the circumstances I will not be explaining why E is a superior answer to A; I hope you understand.. :)

[tanyatomar]

Hi Tim,
i have question on this one.
i chose A the first time. but later became little confused with E and C. following is my line of thought:

we eliminate C beause even if salt is leaching from botttom of ocean or falling dissolved in rain... as long as we can measure some constant increase in it.. we can calculate ocean's age..

about E i rejected it because even the biological activity will be a part of the formulae that wil calculte increase in salt... so even if humans are removing salt from ocean... thta variable will also go in the calculation... basically as long as some known amount os salt is getting added or removed.. we dont care...

but A, if it is proved somehow that the salt concentration is not at some constant level .. and it has been high in recent years than in previous years...=> we cannot make a calculation based on recent years and try to deduce what wud have been the situation some 1000 years ago...


Please correct me if am wrong..
Tanya

[tim]

sounds good!

[jeffwey]

Hi, i understand why answer A makes sense, but i am confused with answer B. If B is negated,then we have "at any given time, the average salt level in the rivers is never consistent" , wouldnt the amount of dissolved salt be different all the time and therefore, it will be inaccurate to calculate amount of salt in the ocean.

am i missing the main point of the question? since the question is asking for amount of salt in the ocean, salt level in the rivers is irrelevant.
But from the question stem it implies that the dissolved salt accumulated in the ocean results from the amount of salt in the rivers..

whats is the best way to correct my thinking method in order to approach the correct answer choice. Thanks so much.

[RonPurewal]

jeffwey, the ocean accumulates salt in aggregate from all the rivers combined. There's no reason to assume an equally proportional contribution from each river.
Some rivers could be very salty, others not. Doesn't matter at all. It's only the total contribution of salt, from all of them, that counts.

As an analogy, think about the total change in a charity's funds from a year's worth of donations, year over year. For this amount to be about the same each year, you clearly don't need every individual donor to contribute the same % of his/her income! Same deal with the salt levels.

[RonPurewal]

oh, and, also --

If B is negated,then we have "at any given time, the average salt level in the rivers is never consistent"

That's much too strong a negation. The negation is just what we know if (b) is false; that would be more like "...may not necessarily have the same salt levels".

Analogy: If a statement says "All bulldogs are stubborn", the negation is not "No bulldogs are stubborn". The negation is just "There are some bulldogs (or even at least one bulldog) that is/are not stubborn."

It's nearly impossible to make rules for this sort of thing. Just think, real-world style, about what it means if thing xxxxx is false.

[jeffwey]

Hi Ron, thanks for the explanation.
And, if we were to negate the answer choices, answer choice A actually weakens the conclusion more than answer choice B does.

[RonPurewal]

Hi Ron, thanks for the explanation.
And, if we were to negate the answer choices, answer choice A actually weakens the conclusion more than answer choice B does.

Choice B has no effect on the conclusion at all. It's completely irrelevant.
If you negate it, it's ... still completely irrelevant.

The reason why it's irrelevant is explained in my post that's 3 posts up from this one.
This post:
post91004.html#p91004