Climatologists believe they know why Earth has undergone a

I had difficulties choosing between C and D.

In my understanding the original statement addresses "cosmic" dust whereas C talks about "atmospheric" dust, hence the two things would be unrelated...

VeritasKarishma : Please help to clarify between C and D.

Can a slight edge of one option over the other be a reason of eliminating one and keeping the other.

Or we need some significant difference/s to choose between right and wrong.

In my view, Option C do lend some support in Climatologists conclusion, as option states that due to dust temperature could drop(even slightly).

Whereas option D, just states a phenomena with no co-relation to Climatologists hypothesis either for or against.

Ice ages began 800,000 years ago.
Earth’s orbit around the Sun has fluctuations that coincide with the ice-age cycles.

Hypothesis: When the fluctuations occur, Earth passes through clouds of cosmic dust that enters the atmosphere; the cosmic dust thereby dims the Sun, resulting in an ice age.
Though cosmic dust clouds are common, the clouds would have to be particularly dense in order to have this effect.

Climatologists’ hypothesis is regarding cosmic dust causing ice ages beginning 800,000 years ago.

Which of the following does not support the hypothesis?

(A) Earth did not pass through clouds of cosmic dust earlier than 800,000 years ago.
Helps establish link between ice age and cosmic dust. Ice ages started 800,000 years ago. So did cosmic dust clouds.

(B) Two large asteroids collided 800,000 years ago, producing a tremendous amount of dense cosmic dust that continues to orbit the Sun.
Again, helps establish link between ice age and cosmic dust. Ice ages started 800,000 years ago. So did cosmic dust clouds.

(C) Earth’s average temperature drops slightly shortly after volcanic eruptions spew large amounts of dust into Earth’s atmosphere.
Helps establish that dust clouds can cause lowering of temperature. So if clouds are thick enough, sun may get blocked and result in massive temperature drop causing ice ages.

(D) Large bits of cosmic rock periodically enter Earth’s atmosphere, raising large amounts of dust from Earth’s surface.
This only says that periodically, dust enters Earth's atmosphere. What impact it has, we don't know. Does it lower temperature periodically, it is not mentioned so it doesn't support our hypothesis. Does it have no impact? If so, our hypothesis may become less likely.
Hence, (D) does not support our hypothesis.

(E) Rare trace elements known to be prevalent in cosmic debris have been discovered in layers of sediment whose ages correspond very closely to the occurrence of ice ages.
It links cosmic dust to ice ages. Supports our hypothesis.

Answer (D)

It isn't about slight edge. It is about only one correct option. Though we do talk about "best option" in Verbal questions, it's mostly relevant for SC and RC questions.

Each of the options except D talks about changes in temperature and/or the ice age cycle that would support the argument.

C suggests that dust causes the earth's temperature to drop, which supports the position that cosmic dust could have a similar impact of reducing earth's temperature.

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I was confused between c and d.
what I understand about the hypothesis of the ice age is 1) cosmic dust enters the atmosphere 2) this leads to DIMMING of the sun
C - still strengthens (2)
but
D just says that cosmic rocks enter which according to the passage are common anyway but cause the ice age ONLY if there dense.

C alone can't be true. The argument says clearly that "the cosmic dust thereby dims the Sun, resulting in an ice age.". It means any other dust source, leading to ice age, will not be supporting hypothesis. Both C and D should be correct answer.

Hello expert,
I read through the thread above, but unable to be convinced. I still can not understand why C is wrong.
Some one said “C says that large amounts of dust (from volcano eruption) have caused drops in the Earth's average temperature. So there's reason to believe that this pattern might be evidenced with cosmic dust as well”, but I think this is far fetched. Although they are both dust, they are totally different things, so how could we infer an effect of cosmic dust from that of volcanic dust? Unless this question requires us to know additional knowledge...

While I think D means: when Earth passes through clouds of cosmic dust, the cosmic dust clouds maybe NOT dense enough to dims the Sun, but cosmic rock occuringnat the same time help to raise large amounts of dust from Earth’s surface, so strengthen the conclusion “have to be particularly dense in order to have this effect”.
Hope experts shed Somme light on this question. Much thanks.

It's supporting evidence—not proof. The connection doesn't have to be completely rigorous.

If we establish that one type of dust cloud causes a lowering of the Earth's surface temperature, that makes it more likely that other types of dust clouds will have the same effect. Yes, they're different types of dust, but physically they're extremely similar. (In terms of the mechanism hypothesized here—partial blocking of the Sun's light—they're identical. Solid particles of a given size will block the same amount of light regardless of their composition.)


In any case, if you have a hard time with choice C, you may be able to solve the problem more easily by directly identifying D as the choice that goes the wrong way.





The hypothesis is that Earth flies through clouds of cosmic dust that are floating around outer space while it's orbiting around the Sun.

If choice D is true, it actually disproves this hypothesis! If D is what's actually happening, then Earth isn't flying through clouds of dust in outer space. Instead, Earth occasionally flies through the path of a BIG rock in outer space, which then crashes into Earth and kicks up bust from Earth itself. That contradicts the idea that the dust is flying around in outer space.

RonTargetTestPrep thanks for your response expert, now I can understand D.
But for C - The connection doesn't have to be completely rigorous, such kind of a choice in GMAT would be an irrelevant choice, thus neuther strengthens nor weakens the conclusion. Is it a different rule since this is a LAST question? Thanks.

the question asks if true, would lend support except,
so the dilemma here is that "is it looking for weakening the conclusion or is this another way of asking a strengthning question.
I selected my practice question only for strengthen ques in cr.
Please give your expert reply on this.

That choice could work on the GMAT. In fact, this problem fits in pretty well with GMAT problems.
Critical reasoning problems on the GMAT rely substantially more on real-world common sense, and far less on formal/rigorous logic, than do their LSAT counterparts. (This is true for very good reasons! Legal reasoning, especially in the use of precedential cases, often does call upon formal logical argumentation. The job of a big business executive, on the other hand, rarely or never uses formal logical reasoning.)

For this type of problem statement, the correct answer is whichever choice does NOT support/strengthen the given argument. That choice could be something that weakens the argument; it could also just be something that has no effect on the argument at all (e.g., because it merely repeats something the argument already says, or because it's irrelevant altogether).

Climatologists believe they know why Earth has undergone a regular sequence of ice ages beginning around 800,000 years ago. Calculations show that Earth’s orbit around the Sun has fluctuations that coincide with the ice-age cycles. The climatologists hypothesize that when the fluctuations occur, Earth passes through clouds of cosmic dust that enters the atmosphere; the cosmic dust thereby dims the Sun, resulting in an ice age. They concede, however, that though cosmic dust clouds are common, the clouds would have to be particularly dense in order to have this effect.

Each of the following, if true, would lend support to the climatologists’ hypothesis EXCEPT:

Following factors lead to regular sequence of ice age:
1. Earth's orbital fluctuations - kind of periodicity
2. Cosmic dust density
3. Drop in temperature on Earth due to dense cloud leading to ice age

I bet no one could easily think of when that regular sequence started i.e. 800,000 years ago could be a valid point that can be tested as a support/weakener of argument.

(A) Earth did not pass through clouds of cosmic dust earlier than 800,000 years ago. - WRONG. Had it been not case then Earth would have undergone a regular sequence of ice ages beginning more than 800,000 years ago i.e. for example 900,000 or 1,000,000 years ago. So, supports.
(B) Two large asteroids collided 800,000 years ago, producing a tremendous amount of dense cosmic dust that continues to orbit the Sun. - WRONG. Highlighted text suggests periodicity.
(C) Earth’s average temperature drops slightly shortly after volcanic eruptions spew large amounts of dust into Earth’s atmosphere. - WRONG. Two factors supported.
(D) Large bits of cosmic rock periodically enter Earth’s atmosphere, raising large amounts of dust from Earth’s surface.
(E) Rare trace elements known to be prevalent in cosmic debris have been discovered in layers of sediment whose ages correspond very closely to the occurrence of ice ages. - WRONG. Scientific evidence of presence of cosmic debris that could have lead to ice age. This is tough as it makes evidence of resultant from those factors.

In D, problem is with "large bits". It can't be dust for sure but more importantly was dust from Earth's surface dense enough. If so, even then it weakens since when such event happened is not sure. It has to be in sync with ice age. It has to be cosmic dust though.

Had a tough time between A and D.

Answer D.

How to choose between C & D.

Let's start by outlining the argument:

• Climatologists believe that the Earth has undergone ice ages because of "fluctuations" that casued it to pass through "clouds of cosmic dust that enter the atmosphere." This dust then dims the sun and causes an ice age.
• The climatologists concede that the clouds of dust would need to be "particularly dense" to cause these ice ages.

Let's now consider (C):


This confirms that dust in the atmosphere can decrease Earth's average temperature. This supports the idea that dust in the atmosphere could lead to an ice age. Eliminate (C).

Let's consider (D):


The argument says that cosmic dust can trigger an ice age. Answer choice (D), by contrast, talks about bits of cosmic rock, which isn't relevant to the argument. While (D) does talk about dust from the Earth's surface, it doesn't tie this dust to lower temperatures, or anything else that might support the idea that dust can cause an ice age.

So unlike (C), (D) doesn't support the idea that dust can lower the earth's temperature, or lead to an ice age. For that reason, (D) is correct.

I hope that helps!