Critical Reasoning

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.