Water vapor evaporated from the ocean contains a greater proportion of oxygen-16 and a smaller proportion of the heavier oxygen-18 than does seawater. Normally, this phenomenon has no effect on the overall composition of the ocean, because evaporated seawater returns to the ocean through precipitation. During an ice age, however, a large amount of precipitation falls on ice caps, where it is trapped as ice.
Which one of the following conclusions about a typical ice age is most strongly supported by the statements above?
[A]. The proportions of oxygen-16 and oxygen-18 are the same in vapor from seawater as in the seawater itself.
[B]. The concentration of oxygen-18 in seawater is increased.
[C]. Rain and snow contain relatively more oxygen-16 than they do in interglacial periods.
[D]. During the ice age, more of the Earth’s precipitation falls over land than falls over the ocean.
[E]. The composition of seawater changes more slowly than it does in interglacial periods.
The question wants an inference about typical ice ages, which are mentioned only in the last sentence, so start there. During an ice age, “however,” precipitation is trapped as ice on ice caps. What’s that “however” in contrast to? It’s the previous sentence, which asserts that “this phenomenon” usually has no effect on the ocean’s composition. Therefore, we can be sure that the trapping of precip on ice caps during an ice age does have an effect on the ocean’s composition. See how that works? Go back over that again—follow the logic. Now, to understand the effect, we have to understand the cause, the “phenomenon.” To what phenomenon are they referring?
It’s in sentence one. Water vapor evaporates from the oceans with more oxygen-16 and less oxygen-18 than seawater contains. And that should allow you to follow the steps through. If all of that water vapor returned to the oceans as precipitation, then there’d be no problem; the proportions of oxygen-16 and -18 would remain pretty much stable, just as sentence two says they do. But if during an ice age, the water vapor doesn’t return to the oceans—if it’s captured by ice—what’s going to happen to the oceans? Seawater is going to have less oxygen-16 (because that water vapor not returning to the oceans is oxygen-16-rich), and will have more oxygen-18 (because water vapor is oxygen-18-poor). That second outcome is choice (B).
(A) contradicts the text which says water vapor and seawater have manifestly different proportions of oxygen -16 and -18.
(C) Even setting aside the fact that the term “interglacial periods” never appears in the stimulus—and hence is a poor prospect for an LSAT inference—(C) has it all bollixed up. The issue is how much oxygen-16 and -18 are returned to the oceans, not how much evaporates from them. There’s no indication that an ice age has any particular impact on the composition of the evaporated water vapor.
(D) contains an irrelevant comparison, and in any case precip over land is outside the scope. All of the author’s interests lie at sea. Me hearties.
(E) may be tempting if you start picturing those icecaps melting slowly. But the author’s sole interest is in comparing the composition of normal seawater and ice-age seawater, and the rate of change is never taken up. For all we know, seawater’s composition changes at the very same rate whether there’s an ice age or no.
Don’t just rush through your reading of each question stem. Milk it for all possible help, all possible hints. Here, you’re told it’s an inference about ice ages, which directs your attention to the last sentence as the most important one. You’ll waste less time, and get less distracted, if you let the question stems help you out.
Stick closely to the topic and scope of the stimulus when seeking an inference. The further a choice departs from that which the author takes up, the more likely it is that that choice is dead wrong.