Water vapor evaporated from the ocean contains a greater proportion of

Premise: no effect on the overall composition of the ocean, because evaporated seawater returns to the ocean through precipitation
Premise: During an ice age, however, a large amount of precipitation falls on ice caps, where it is trapped as ice.

the premises tell us that although the overall composition of ocean does not change BUT during an ice age it does because a large portion of precipitation (more O-16) is trapped as ice on ice caps.

this must mean that the concentration of O-18 is now increased.

B is correct

water vapor from ocean O16 > O18
water vapor from sea water O16 > O18---------->TRAP.
Water vapor evaporated from the ocean contains a greater proportion of oxygen-16 and a smaller proportion of the heavier oxygen-18 than does [water vapor from] seawater

(A) The proportions of oxygen-16 and oxygen-18 are the same in vapor from seawater as in the seawater itself. >> Out of scope / Cannot be inferred.
(B) The concentration of oxygen-18 in seawater is increased. >> O16 evaporates causing O18 to increase in seawater. O16 is not returned. Answer
(C) Rain and snow contain relatively more oxygen-16 than they do in interglacial periods. >> Rain is alright.But snow is wrong. OUT
(D) During the ice age, more of the Earth’s precipitation falls over land than falls over the ocean. >> Out of scope
(E) The composition of seawater changes more slowly than it does in interglacial periods. >> Out of scope

Hey All

Now a question -

"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."

It means water vapor from ocean has O16 > O18 obviously
But the second half.

Is it comparing water vapor 's concentration from ocean with sea water ?

or is it comparing water vapor from ocean w/ water vapor from sea water ?

Can some one answer this pls?

critical thinking

actually this is a comparison of composition ... between water vapor (of the ocean) and seawater

e.g. let's assume
water vapor (from the ocean): 75% O-16, 25% O-18
seawater: 65% O-16, 35% O-18

now the argument is comparing the oxygen concentration between the two.
i.e. water vapor (from the ocean) has greater composition of O-16 than does saltwater -----> equal amount of seawater btw

i hope i made some sense!

Your explanation is acceptable with regard to the OA.

But I think, the text is a bit confusing to find the right answer. A, C, D, E are clearly out.

And let's consider the OA in the light of the main text.

The first premise is "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."
This premise separates the seawater from the ocean water. On this basis the second premise - "Normally, this phenomenon has no effect on the overall composition of the ocean, because evaporated seawater returns to the ocean through precipitation." - seems clearly ambiguous because of the use of the word "return".

Moreover, one may, because of the words " precipitation falls on ice caps", assume two cases, i.e. either water circulation from the ocean to the sea or vise versa. Herewith the reader can also be trapped in so far as the ocean water does usually not become icy (here the ice age is not an absolute evidence to disprove this fact and otherwise as in the GMAT-format the test-taker is not assumed to posses the knowledge of the special subject, as here the case relates to the geography), the same is true under conditions regarding the seawater.

Take my evaluations into consideration then even B is also not a proper ans. choice.

I would say it a bad question because of such internal contradictions in its construction and ambuqity of its content.

B is correct.

Logic goes like this: Water evaporates with fewer O 18 molecules than O 16. In ice age, O 16 molecules are trapped, leaving relatively more heavy O 18 molecules in seawater than before.

Hi gatreya14 - In response to your question, LSAT reasoning questions are very good prep for GMAT CR section. LSAT as a whole is heavily focused on logic and would therefore be a great resource to utilize in preparation for this exam. I'm not saying to go out and start purchasing LSAT books, but utilize these questions when you are feeling shaky on this topic, especially if you're working in the 700-level questions. Have faith though. Keep practicing the 700 level and you will get better

so O-16 is being evaporated, but not going back in the seawater, therefore, there will be more O-18 left in seawater during the ice age than during normal times.


(A) The proportions of oxygen-16 and oxygen-18 are the same in vapor from seawater as in the seawater itself.
doesn't explain anything. since same amount is evaporated vs. what is left - then nothing should be changed though...

(B) The concentration of oxygen-18 in seawater is increased.
aha - so more O-18 is in the seawater, as O-16 is evaporated and not coming back...looks like a good answer.

(C) Rain and snow contain relatively more oxygen-16 than they do in interglacial periods.
seems tempting...but interglacial periods - what is that? it speaks about different period than we are looking for..out

(D) During the ice age, more of the Earth’s precipitation falls over land than falls over the ocean.
sure? 2/3 surface of earth is ocean - so i don't think it is true...

(E) The composition of seawater changes more slowly than it does in interglacial periods.
again - interglacial periods...a period that we are not interested in...

This is an inference question, in which we must determine which of the following answers can be concluded from the above.

What do we know? We know that the ocean contains both oxygen-16 and oxygen-18. However, water made from oxygen-16 evaporates more readily. During normal times, this precipitation falls on the land or in snow packs and then melts in the spring to return to the oceans.

In an ice age, however, more and more ice accumulates on land. This ice is made up mostly of water containing oxygen-16. Meanwhile the concentration of oxygen-18 increases in the ocean because oxygen-16 is reduced and never replaced.

This information best supports answer choice (B).

Present day : more oxygen 16 is evaporated, but it comes back to sea . Composition of sea water wrt 0xygen 16 and 18 remains unchanged

Glacial age : More oxygen 16 is evaporated , but it does not back to ocean , rather its trapped in ice. Thus composition of Oxygen 18 increases in sea water . Thus B is the derived inference