LSAT 117 | Section 4 | Logical Reasoning: Q17

LR Question 17 Explanation

QUESTION TEXT: Climatologists believe they know why Earth …

QUESTION TYPE: Strengthen – EXCEPT

CONCLUSION: Ice ages happen when fluctuations in Earth’s orbit make Earth pass through dense cosmic dust clouds, which dims the Sun.

REASONING: Earth’s orbital fluctuations line up with ice-age cycles.

ANALYSIS: This is a Strengthen EXCEPT question. So four answers support the cosmic-dust hypothesis. One answer does not. That one is correct.

The hypothesis is: orbital fluctuation → dense cosmic dust → dust enters atmosphere → sunlight dimmed → ice age

A strengthening answer could support any part of that chain.

___________

1. This helps with the timeline. The ice-age cycles began around 800,000 years ago. If Earth didn’t pass through cosmic dust clouds before then, that makes it more plausible that the onset of these dust encounters coincided with the beginning of the ice ages.
2. The scientists admit the need for especially dense cosmic dust clouds. This answer provides exactly that: a major source of dense cosmic dust beginning around 800,000 years ago.
3. You might have chosen this answer because it’s about volcanic dust, not cosmic dust. But the hypothesis only requires: dust in the atmosphere can cool Earth. If volcanic dust can lower Earth’s temperature, then it becomes more plausible that cosmic dust in the atmosphere could also cool Earth enough to contribute to an ice age. So C makes the mechanism more likely by showing that other dust has a cooling effect.
4. CORRECT. This doesn’t support any link in the hypothesis. Does Earth become cooler after? Don’t know. Does this occur during ice ages? Don’t know. Is this the same as passing through dense cosmic dust clouds? Nope. This just tells us that cosmic rocks sometimes stir up dust from Earth’s surface, which doesn’t help support anything.
5. If elements associated with cosmic debris appear in sediment layers that closely match the timing of ice ages, then there is evidence that cosmic material was present when the ice ages occurred. That supports the connection between cosmic dust and ice ages.