Critical Reasoning

ov25 wrote:imo A is still a strong contender...

herez my take,

E) None of the salts carried into the Earth’s oceans by rivers are used up by biological activity in the oceans

says biological activity uses up the some salt (notice the authors did not use the word 'unusually') here... lets example

5 units every century and 1 unit is used up - lets say 3 centuries lapsed; salt deposited=15units, current levels=12units. So the "maximum" prediction could be 3 because, the current levels exceed 10. So this 'could' accurately predict the age as 3 centuries

However in the case of A, unusually high levels could distort the prediction off by couple centuries...
yr 1=5, yr2=10 (unusually high); yr 3=5; Total = 20 units
Prediction = 4yrs and actual = 3yrs.

jainrahul1985 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.
Confused b/w A and E . Please suggest

ov25 wrote:Jainrahul wrote: Please notice how you keep using the word could.

Where ever I referred to could, I was basing on "the maximum age of the Earth's oceans can be accurately estimated" part of the stimulus.

lunarpower wrote:

GMATGuruNY wrote:

ov25 wrote:Jainrahul wrote: Please notice how you keep using the word could.

Where ever I referred to could, I was basing on "the maximum age of the Earth's oceans can be accurately estimated" part of the stimulus.

Having finished my coffee, I'm starting to have a change of heart. As noted by Stuart, E is too extreme. It doesn't pass the negation test. Negated, E would say:

Some of the salts carried into the Earth's oceans by rivers are used up by biological activity in the oceans. (The opposite of none is some.)

Some of the salts could be a very, very small amount. If only a very small amount of the salts are used up, the conclusion of the argument remains valid: we could still deduce the age of the oceans.

Answer choice A, however, does pass the negation test. Negated, A would say:

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

If the statement above is true, we can't use data for the past hundred years to deduce what happened during previous centuries.

So the credited response should be A.

Sorry for the confusion! I should know better than to answer questions before I've been adequately caffeinated.

(Please note that to avoid further confusion I've edited my initial response above.)

this is all good, but the problem with (e) is even more fundamental than picking at the word "none".

specifically, we're talking about increases in the oceans' salt levels, not the actual values of the salt levels themselves.
therefore, if biological activity is using up some of the salts, the increases will just be smaller -- but they'll be smaller across the board, thus NOT detracting from our ability to use them to estimate the maximum age of the ocean.
an estimate of the ocean's age could only be affected if biological activity used up ocean salts at erratic, highly fluctuating rates -- a highly unlikely outcome. if biological activity uses up ocean salts (especially over the extremely long time periods referenced in the passage), then it is almost certain to use up those salts at a very regular, predictable rate, meaning that the resulting estimates would not be affected.

as an analogy, let's say that biological activity uses up 50% of the marine salt.
this just means that all of the rates of increase -- across the board -- will just be half of what they would be without such activity. this won't change any of the estimates of the age of the ocean, since those estimates will just be based on rates of increase that are half as great.

if choice (a) is not true, on the other hand, then the data for the past hundred years will be wholly unrepresentative of the overall collection of data, and will thereby be useless for statistical inference. so you definitely need that assumption.

heyabhi wrote:Whats wrong with B ? Isn't that an assumption too ?