LSAT 1 | Section 3 | Logical Reasoning: Q11

LR Question 11 Explanation

QUESTION TEXT: Water vapor evaporated from the ocean contains a greater …

QUESTION TYPE: Most Strongly Supported

FACTS:

1. Water vapor that evaporates from the ocean has more oxygen-16 and less oxygen-18 than seawater.
2. Normally, that vapor returns to the ocean via precipitation, so there is no net change in the ocean’s composition.
3. During an ice age, much of that precipitation gets trapped in ice caps instead of returning to the ocean.

ANALYSIS: If vapor that is rich in oxygen-16 leaves the ocean and doesn’t return (because it’s trapped in ice), the ocean loses oxygen-16 and keeps more oxygen-18.

So, during an ice age the relative concentration of O-16 decreases and O-18 increases in seawater.

___________

1. We’re explicitly told that vapor has more O-16 and less O-18 than seawater, so this is clearly false.
2. CORRECT. This is exactly what we can infer. O-16 leaves via evaporation and gets trapped in ice, so the ocean becomes richer in O-18.
3. We’re not told anything about how precipitation changes between different time periods. This is too speculative.
4. We’re told that precipitation falls on ice caps, but nothing about the overall ratio of land vs. ocean precipitation.
5. We’re not given any information comparing the rate of change.