vjsharma25 wrote:
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:
(A) Earth did not pass through clouds of cosmic dust earlier than 800,000 years ago.
(B) Two large asteroids collided 800,000 years ago, producing a tremendous amount of dense cosmic dust that continues to orbit the Sun.
(C) Earth’s average temperature drops slightly shortly after volcanic eruptions spew large amounts of dust into Earth’s atmosphere.
(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.
Source : LSAT PrepTest 44, Q17
Strengthen the Argument (All EXCEPT)A choice that fails to strengthen an argument either contradicts it or is irrelevant to it.
Four of the choices support the idea that extremely
dense cosmic dust periodically
blocking the sun has been the cause of each of the ice ages that Earth has suffered beginning 800,000 years ago. We need to go into the choices confident that (1) there’s a 4:1 chance—that is, a strong likelihood—that each of these choices does
support the cosmic dust idea, and (2) one and only one of them either contradicts that hypothesis or falls outside its scope. Be bold as you consider each:
(A) Suppose the first cosmic dust clouds appeared 800,000 years ago. That would be a “co-incidence” in the most formal meaning of the term: two things happening together. As we know, correlation doesn’t prove causation, but a strengthener doesn’t have to prove the point; it just has to bolster it. (A) does so by connecting the first appearance of a phenomenon to the hypothesized cause of the phenomenon.
Eliminate.
(B) The appearance, 800,000 years ago, of a huge (and thereafter constant) cosmic dust cloud certainly supports the hypothesis, as surely as the absence of any cosmic dust around that time would weaken it.
Eliminate.
(C) draws a palpable connection between cold temperatures and atmospheric dust. Sure, it’s only a “slight” temperature drop, but maybe that’s enough to cause an ice age.
Eliminate.
(D) asserts that Earth
suffers cosmic dust clouds. Well, we already knew that; “
cosmic dust clouds are common,” say the climatologists. (D) does NOTHING to relate those clouds to any of the factors allegedly contributing to the periodic ice ages, so (D) is the “odd man out” we seek.
We can be quite confident that (E) is another
strengthener, and so it is, connecting cosmic debris—the result of cosmic dust—to the periodic nature of ice ages.
Quote:
The conclusion of this argument is that the cause of ice ages is the earth passing through cosmic dust clouds, which in turn is caused by fluctuations in the Earth's orbit around the sun. (I always thought that!). The climatologists base their argument on the fact that the occurrences of ice ages, starting 800,000 years ago has coincided with fluctuations in the Earth's orbit around the sun. The climatologists note that the clouds would have to be quite dense for this all to be true.
We're looking for an answer that does NOT strengthen the argument. (D) introduces the fact that cosmic rock periodically enters the earth's atmosphere and kicks up dust. This WEAKENS the argument in that it raises the possibility that there's another reason for the dust that causes the ice ages: it's the cosmic rock dust, NOT the fluctuations! However, there are problems with this answer as a weakener in that we'd need to do a lot of "work" to make (D) weaken: periodically would have to coincide with the ice-age cycles, the dust that is kicked up would have to be dense, and would have to reach the atmosphere - though, as one poster below suggests, perhaps we should consider anything above the earth's surface to be atmosphere! But, let's not lose sight of what we're looking for - something that does not strengthen, and regardless of whether (D) weakens or is neutral, it DOESN'T strengthen the argument.
(A) supports the connection between the clouds and the ice ages: of course there were no ice ages before then, we didn't float through any dust clouds!
(B) establishes that there is indeed a dense cloud around the sun, something the climatologists conceded needed to be true -- and answers the nagging question of why the ice ages only began in the year 800,000 b.c.e.
(C) supports the idea that dust causes a lower temperature, strengthening the argument.
(E) provides more evidence that Earth passed through a cosmic cloud around the time of the ice age.