The mastodon, a large mammal resembling today's elephant,

TIP: When answering a strengthening or assumption question, identify the single sentence that is the conclusion, in your head, add the word 'because', and then read the answer choice. The resulting sentence should flow logically.[/i][/b]

Try this with answer choice B, and you should 'hear' the problem:

"The disappearance of wetlands caused the mastodon's extinction because the draining of the glacial wetlands left a new topography." You still have to explain why topography caused the extinction of the mastodons.

Now 'listen' to D:
"The disappearance of the wetlands caused the mastodon's extinction because the mastodons fed on wetland plants." The sentence makes explicit the mastodon's reliance on the wetlands.

If you have to add an extra explanation in your head to make the sentence make sense, then there's a problem.

The trouble with B is that it only supports the idea that the wetlands did indeed drain. That does nothing to support the idea that the disappearance of the mastodon was caused by this draining. We don't know whether this event is even in doubt. (Perhaps it's accepted science.) Even if it were, establishing its existence wouldn't do much for our causal argument.

D, on the other hand, shows us why the draining of the wetlands could actually have driven the mastodon to extinction.
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I'm afraid the alternative you're describing for D is impossible. Wetlands are tracts of land where the soil is saturated with water. If the wetlands receded, that would mean the water plants there could not grow, even if somehow a river ran through that area.

More broadly, it's important to recognize that we are never looking for a "100%" strengthen. We're simply looking for something that gives us more reason to suspect that the author's reasoning is correct. In this case, we're not even facing a pure strengthen question. It's more of a discrepancy question--why do some scientists hold this contrary view? We need something that would explain why they would prefer this explanation to the old one. Simply establishing that the wetlands disappeared doesn't really do anything for the logic of the argument. Presumably, we also know that the evergreen forests disappeared. Lots of other changes surely happened, too. What we need to explain is why this particular change would have led to the mammoth's extinction. That's what D makes clear. It doesn't need to prove the scientists right--again, the correct answer will almost never do that--it just needs to show why their explanation makes sense.
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Our answer should reaffirm the belief that the extinction of the poor animals was caused by draining of glacial wetlands

(A) When the Ice Age ended, many forests died off, endangering the food supply for herbivores that fed primarily on evergreens: okk....but it does not relates to draining of wetlands as such
(B) As glaciers receded at the end of the Ice Age, glacial wetlands drained, leaving a new topography of hills and plains: it is basically working in opposite direction, i.e. according to this option, the drained wetlands should have given life instead of taking it
(C) Bacteria found in mastodon remains are similar to bacteria found in the remains of species that are known to have fed on evergreens: so what??
(D) The mastodon's diet consisted primarily of water plants such as marsh grasses and water lilies: Bingo, this is our answer as with those water plants gone, the poor animals got extinct
(E) Evergreen forests were inhabited by many species of large mammals until the end of the Ice Age: not good at all

This question definitely deserves more attention, in particular, the reasoning behind the rejection of option B.

egmat GMATNinja

D, since draining of the glacial wetlands would lead to decreased food supply for the mastodon.

thanks. why isn't B ? Pls elaborate.

It cannot be B because it does does give a relationship between the drained glacial wetlands and mastodon, we are not suppossed to assume anything, D is more elaborate and logical.

we are not suppossed to assume anything -----> I didn't get the point. Pls elaborate.

There are 2 theories here. And the answer will weaken the first and strengthen the latter.

hmm.B!

disappearance of evergreen forests could be one possible effect of new topology caused by the draining of glacial wetlands

Good explanation SaraiGMAXonline

Nice method to double check strengthen question and fish out last of the contenders.

I get that, but B, maybe not in the strongest way, but still 100% strengthens the conclusion. The question talks about "Discovery", meaning that the receding glacier wetlands was not factual truth before the discovery mentioned in option B.

Regarding option D, I see the causal link, but it's not 100%, maybe water plants thrive even better when glacier wetlands diminish and rivers and other types of water sources replace them, who knows.

So I look at them this way -- B is a weaker strengthened but is 100% working. D is a stronger strengthened but needs further assumptions to link to the conclusion, so its more remote strengthener.

Conclusion: The mastodon's extinction was caused by the draining of glacial wetlands.

Pre-thinking: The mastodon's food must be depended on glacial wetlands. So, once glacial wetlands had drained then mastodon got extinct.

Which of the following, if recently discovered, would help to account for the change in the accepted explanation of the mastodon's extinction?

(A) When the Ice Age ended, many forests died off, endangering the food supply for herbivores that fed primarily on evergreens. -> It is not as per conclusion. Incorrect.
(B) As glaciers receded at the end of the Ice Age, glacial wetlands drained, leaving a new topography of hills and plains. -> But, new topology might or not affect mastodon. Incorrect.
(C) Bacteria found in mastodon remains are similar to bacteria found in the remains of species that are known to have fed on evergreens. -> It is not as per conclusion. Incorrect.
(D) The mastodon's diet consisted primarily of water plants such as marsh grasses and water lilies. -> So, if wetlands drained then the primary foods won't be available for mastodon. It is the reason for their extinction. Let's keep it.
(E) Evergreen forests were inhabited by many species of large mammals until the end of the Ice Age. -> Irrelevant.

So, I think D.

Can anyone explain what is wrong with A ?

We need to explain that the draining of the glacial wetlands caused the mastodon extinction.

Option B simply says that a new topography formed, however there is no indication of whether it was good or bad for the mastodons. Only if we make an additional assumption that it was bad can we use this option to explain the extinction.

Option D, on the other hand, is saying that the mastodons PRIMARILY ate marsh grasses and water lilies. In other words, these wetland plants made up most of their diet, without which they are unlikely to have survived. Fairly airtight.

Therefore, using option D, we can explain the extinction with more certainty than we can with option B.

I chose B because it was in line with my pre-thinking - what if the draining of glacial wetlands caused the disappearance of evergreen forests? Then the real cause would be the draining of the wetlands.

Can someone please explain how I am wrong here? Thank you!

I guess you overlooked one part of the passage. Please see below:

Scientists once attributed the mastodon's extinction to the disappearance of evergreen forests at the end of the Ice Age.
More recently, however, scientists have rejected that theory, hypothesizing instead that the mastodon's extinction was caused by the draining of glacial wetlands.

Note here that the scientist have disregarded their earlier theory. They now believe that disappearance of evergreen forests didn't play any role in extinction.
So we have to find an option that suggests that the survival of mastodon depended on wetlands AND NOT on evergreen forests.
Once you are clear with this understanding try to solve the question again.
In your prethinking you still believed that the cause of extinction was still disappearance of evergreen forests, and indirectly glacial wetlands.

Understanding the argument - We must strengthen the new hypothesis that "the mastodon's extinction was caused by the draining of glacial wetlands." We have to strengthen this cause.
Option Elimination -

(A) When the Ice Age ended, many forests died off, endangering the food supply for herbivores that fed primarily on evergreens. - Strengthen the earlier explanation while we need the strengthener for the new hypothesis.
(B) As glaciers receded at the end of the Ice Age, glacial wetlands drained, leaving a new topography of hills and plains. - So was the new topography suitable or not suitable for Mastadons - we don't know.
(C) Bacteria found in mastodon remains are similar to bacteria found in the remains of species that are known to have fed on evergreens. - Strengthen the earlier explanation while we need the strengthener for the new hypothesis.
(D) The mastodon's diet consisted primarily of water plants such as marsh grasses and water lilies. Perfect.
(E) Evergreen forests were inhabited by many species of large mammals until the end of the Ice Age. - Strengthen the earlier explanation while we need the strengthener for the new hypothesis.