#4 - Scientists analyzing air bubbles that had been trapped

[Sophia123]

Hi,

I got this question correct, but I struggled with justifying why it was correct other than the fact that the other answer choices seemed less correct. My main hesitation with this answer was that it said that the sediment... near Antarctica didn't reflect an increase in diatoms made me think that it was not correct either because the question is referencing Antarctica, not some area near Antarctica.

Thank you in advance!

-Sophia

[Francis O'Rourke]

Choice (D) tells us that when we look at the ocean floor near Antarctica, there is no sign that there was an increase in diatom shells during the last ice age, and thus no diatom increase at that time. This directly attacks the scientist's hypothesis that the ferrous material promoted population growth in Antarctic algae, such as diatoms, during the last ice age.

The question you have is about the word 'near.' It is an ambiguous phrase, but it indicates that the sediment samples came from the ocean floor right next to the land mass of Antarctica up to a mile or so from the land mass. Since Algae are aquatic, this is the closest area you would look for evidence of Antarctic algae.

[willmcchez]

My issue with D being incorrect probably stems from my pre-phrasing. Help me with this:

The conclusion states "The scientist hypothesized that the ferrous material, which had contained atmospheric dust, had promoted a great increase in the population of Antarctic algae, such as diatoms."

I bracketed the conclusion, but then marked out the emboldened phrase above. My immediate thought was that "such as diatoms" indicated that diatoms were just one variety of algae that increased in population during the ice age. There is no way to say that diatoms themselves, as one variety of algae, could be assigned any degree of responsibility for the carbon dioxide absorption. It doesn't state that they were the most prominent or played a significant role.

Thus, when the correct answer eludes to the fact that because there wasn't an increase in diatom during the last ice age that it wasn't algae that was responsible for the reduced carbon dioxide, I marked it off because of my logic above. Just because there wasn't an increase in diatom shells doesn't mean that other types of algae weren't responsible.

I selected C because I thought that it gave justification for something other than "ferrous materials" (in this case, other minerals) being responsible for the promotion of algae population.

Thanks in advance!

[Adam Tyson]

Good analysis, willmcchez! The issue you are having is that answer D doesn't disprove the hypothesis, because diatoms are just one type of algae, and perhaps they didn't increase but other types did. Absolutely true!

The problem is that we were not asked to disprove the hypothesis, but only to weaken it. If the hypothesis was correct, we should expect an increase in ALL types of Antarctic algae, including diatoms. The author didn't just mention them by accident - he named them as being a type of algae that the hypothesis suggests should have increased. Answer D tells us that we are missing some evidence that we would probably expect to find if diatoms had increased. That is enough to make us stroke our beards thoughtfully as we say "huh, that's odd", and we start to doubt the accuracy of the hypothesis, even if it's only a small doubt. That's enough! Doubt is the goal in weaken questions!

Since none of the other answers give us any reason to doubt the hypothesis, and answer D at least is something that makes us go "huh", it's the best of the bunch and the credited response.

Answer C gives us other stuff in the air bubbles, but the ferrous material is still there. So what if there's other stuff? Why should we believe that the other stuff, rather than the ferrous materials, are responsible? That's my response to C - "so what?" rather than "huh, that's odd" - and that's not the response we want for a weaken answer. Only by adding extra help to that answer ("hey, maybe that other stuff did it instead of the ferrous stuff?") does it start to introduce doubt, and we definitely do not want to be in the habit of helping answer choices, not when the authors are already doing such a good job of confusing and distracting us with attractive wrong answers!

Take another look at this and other weaken questions in light of the "element of doubt" approach, and see if they don't start to get a little easier and more obvious for you. Good luck!

[LSAT2018]

Would answers (B) and (E) be incorrect because they refer to the recent times, thus making it beyond the scope of the argument concerned with the last ice age?

Thanks in advance!

[Adam Tyson]

Not necessarily, LSAT_2018, although that might make us a little skeptical. For example, if answer B had instead said "Computer models suggest that a large increase in ferrous material today would not greatly promote the growth of oceanic algae", then I would at least consider that as an attack on one of the premises that could weaken the argument.

As it stands, B is out because if it does anything at all, it strengthens the argument, by showing that if the alleged cause were to occur, the alleged effect would perhaps also occur.

Answer E is out because the issue isn't whether some types of algae are or are not harmed by ferrous material, but whether some types (perhaps not the same types) are promoted by it. So what if some algae gets harmed, or doesn't get harmed? If some algae increases dramatically, that would help, and if none would, that would hurt.

The timeframe is worth thinking about, but it isn't dispositive by itself. We'd need more context than just "that was then, this is now" to eliminate those types of answers.

[psalom625]

Wouldn't the fact there are additional materials alongside the ferrous material indicate a possible alternative cause for the increase of diatoms?

Is answer choice D more correct in that it adheres more to the question's parameter of "most seriously undermine"?

[Rachael Wilkenfeld]

Hi Psalom,

I assume your first question is referencing answer choice (C). The problem with that answer choice is that there's no connection made between other minerals and algae. It just states there were other minerals present. This doesn't impact the argument without some sort of a link to the rest of the argument. In fact, we'd anticipate that there were many additional minerals present in any land mass/ocean floor. What we would need is information on how those minerals would impact the growth of algae.

Answer choice (D) however does weaken the argument by stating that something we'd expect to see if the scientists were correct--diatoms--we do not see. There could still be other alternatives algae, however the lack of the one suggested would weaken the argument at least a bit.

Hope that helps!
Rachael

[whardy21]

I picked answer choice C. I chose C because my reasoning led my to believe that answer choice presented an alternate cause or the stated effect. Therefore, would weaken the casual conclusion that ferrous material promotes an increase in the population of antartic algae. Please detail to me why the logic to answer choice C is incorrect.

[George George]

@whardy21

First, please note that your prephrase here was a good one!

Answer (C) just says that there were "other minerals," but doesn't tell us what effect those minerals may or may not have had. In general, answers like this are vague - you don't have enough information in order to make an assessment as to whether they're alternate causes or not. For (C) to be correct, it would have to say something like: "The dust found in the air bubbles trapped in Antarctic ice contained other minerals known to promote algae formation in addition to the ferrous material." Note that, on its own, you'd have to make this assumption, and hence the answer choice is incorrect.