Rainwater contains hydrogen of a heavy form called deuterium. The

Question

Rainwater contains hydrogen of a heavy form called deuterium. The deuterium content of wood reflects the deuterium content of rainwater available to trees during their growth. Wood from trees that grew between 16,000 and 24,000 years ago in North America contains significantly more deuterium than wood from trees growing today. But water trapped in several North American caves that formed during that same early period contains significantly less deuterium than rainwater in North America today.

Which of the following, if true, most helps to reconcile the two findings?

(A) There is little deuterium in the North American caves other than the deuterium in the water trapped there.

(B) Exposure to water after a tree has died does not change the deuterium content of the wood.

(C) Industrialization in North America over the past 100 years has altered the deuterium content of rain.

(D) Trees draw on shallow groundwater from rain that falls during their growth, whereas water trapped in caves may have fallen as rainwater thousands of years before the caves formed.

(E) Wood with high deuterium content is no more likely to remain preserved for long periods than is wood with a low deuterium content.

ID - CR03749

Deuterium in Water

Step 1: Identify the Question

The question asks you to reconcile the two findings so this is an  Explain the Discrepancy  question.

Step 2: Deconstruct the Argument

Trees 16K-24K = more D

Cave water same time = less D

Step 3: Pause and State the Goal

On Discrepancy questions, your goal is to find a piece of information that makes the findings in the argument less surprising. It often helps to restate exactly what is surprising in the argument in your own words; it is strange that the trees and water from caves of the same age have different levels of deuterium. What would resolve this discrepancy?

Step 4: Work from Wrong to Right

(A) The argument is comparing deuterium from water in trees and caves. The fact that there is little deuterium in the caves aside from the water found there does not explain the different levels observed in caves and trees.

(B) This answer actually makes the finding more surprising. If the deuterium content increased after trees died, this might explain the higher deuterium content in wood. But if the level does not change, then it’s still unclear why the trees and caves from the same time period have different levels of deuterium.

(C) The argument compares the deuterium content in wood and cave water from several thousand years ago; anything that happened in the last 100 years is not relevant.

(D) CORRECT . This answer suggests that even though the trees and caves are of the same age, the water in the trees and the water in the caves may be of different ages (cave water may be thousands of years older). If the water is from different time periods, different levels of deuterium are less surprising.

(E) This information makes the findings even more puzzling. Only wood that remains preserved can be analyzed thousands of years later, so if high deuterium wood was more likely to be preserved that could explain the higher deuterium levels in wood. But if, as this answer states, wood with any level of deuterium can remain preserved, then it’s still not clear why the trees and caves from the same time period have different levels of deuterium.

GMATRockstar · Accepted Answer

https://www.youtube.com/watch?v=8gGl3M0KbSM

egmat · Answer

Passage analysis  
 Rainwater contains hydrogen of a heavy form called deuterium. 
 
Deuterium is a heavy form of hydrogen.
 
 The deuterium content of wood reflects the deuterium content of rainwater available to trees during their growth. 
 
The amount of Deuterium present in wood of trees shows how much deuterium was present in the rainwater that was used by these trees during their growth period.
 
 Wood from trees that grew between 16,000 and 24,000 years ago in North America contains significantly more deuterium than wood from trees growing today. 
 
A significantly high amount of deuterium was found in the wood from trees belonging to the period between 16,000 and 24,000 years ago in North America.This amount is significantly higher than the amount of deuterium found in wood from present day trees.  
We can infer that the rainwater in the period between 16,000 and 24,000 years ago in North America had much higher amount of deuterium than the rainwater today 
.  
 But water trapped in several North American caves that formed during that same early period contains significantly less deuterium than rainwater in North America contains today. 
 
Several North American caves that were formed during that same period (16,000 to 24,000 years ago) have water trapped in them.
Contrary to expectation, this trapped water has significantly less deuterium content than found in rainwater in North America today.

Prethinking 
 Thought process 
We know:

Rainwater contains deuterium. 
 The deuterium amount present in the wood of a tree tells us how much deuterium was present in the rainwater that was used by that tree. 
 Wood from trees that grew from 16,000 – 24,000 years ago show much more deuterium that the wood from present day trees. 
 This means the rainwater at that time must have contained a similarly high deuterium content. 
 The water trapped in many caves that formed during the same period show much less deuterium as compared to rainwater today.

If the caves belong to the same period as the trees under consideration, then the deuterium content of the wood and the water trapped in the caves should be the same. But the first is higher than the second.

This is the paradox in the passage that we have to resolve.
 Clearly the same rainwater could not have been the source of the water absorbed by the trees and the water trapped in the caves.

The trees absorb rainwater from the ground when the rainwater falls on the ground and is absorbed by it. So, the deuterium content of the wood shows the accurate deuterium content.

But the caves and the water trapped therein could have another explanation. The caves may have formed during the same period. But it is possible that the water got trapped into the rocks and the ground long before the caves started forming. And the rainwater at that time had less deuterium than the rainwater available to the trees later.

Thus, water trapped in caves may have fallen as rainwater thousands of years before the caves formed.

This explains the paradox.

Answer Choice Analysis

Option A

The deuterium present in the rocks of the caves is not being investigated by the argument. The trapped water deuterium content is the main concern. The comparison in this option, therefore, does not throw any light on the paradox.
Thus, this is not the correct answer choice.
 
Option B

This supports the data given in the passage that the amount of Deuterium present in wood of trees shows how much deuterium was present in the rainwater that was used by these trees during their growth period. Even after the tree is dead, its deuterium content remains the same. Though this confirms the premise, it does not give any insight into the paradox.
Thus, this is not the correct answer choice.

Option C

This explains the reduced deuterium content in rainwater today but fails to explain the difference in the deuterium content of the trapped water in the caves and wood from trees, both belonging to thousands of years ago.
Thus, this is not the correct answer choice.

Option D

This is in line with our pre-thinking.
Thus, this is the correct answer choice.

Option E

The wood under discussion in the passage must have remained preserved for it to be investigated today. This is despite whatever deuterium content it has. The comparison does not serve any purpose in resolving the paradox.
Thus, this is not the correct answer choice.

Skywalker18 · Answer

Wood from trees that grew between 16,000 and 24,000 years ago in North America contains significantly more deuterium than wood from trees growing today. But water trapped in several North American caves that formed during that same early period contains significantly less deuterium than rainwater in North America today.

Pre - thinking - both the water in trees and North american caves are not from the same period . Just because the caves were formed between 16,000 and 24,000 years ago , the water trapped in the caves need be from the same period . 
Or maybe wood from trees with high deuterium content is likely to remain preserved .
Paradox -

A) There is little deuterium in the North American caves than the deuterium in the water trapped there. Does not explain 
B) Exposure to water after a tree has died does not change the deuterium content of the wood. Does not explain
C) Industrialization in North America over the past 100 years has altered the deuterium content of rain. Irrelevant 
D) Trees draw on shallow groundwater from rain that falls during their growth, whereas water trapped in caves may have fallen as rainwater thousands of years before the caves formed. 
E) Wood with high deuterium content is no more likely to remain preserved for long periods than is wood with a low deuterium content.
 This states the opposite of what we are looking for .
If wood with high deuterium content is more likely to remain preserved for long periods than is wood with a low deuterium content , then this might explain the paradox .

Correct answer D

HiLine · Answer

I expected the answer to say something to the effect of "trees and caves use different kinds of water" or "trees and caves use water differently". Answer choice D fits the profile. :-D

abrakadabra21 · Answer

Argument says :

D content of wood = deuterium content of rainwater available to trees during their growth.

water trapped in several North American caves < rainwater in North America
Wood from trees that grew between 16,000 and 24,000 years ago in North America > wood from trees growing today
this should not be the case

Assumption is that water trapped in several North American caves = deuterium content of rainwater available to trees during their growth.

So if we break this assumption, we would be able to figure out discrepancy in finding.

D) Trees draw on shallow groundwater from rain that falls during their growth, whereas water trapped in caves may have fallen as rainwater thousands of years before the caves formed.

D says = D content of tree = rainwater and
water trapped in caves may have fallen as rainwater thousands of years before the caves formed.

so water trapped in cave doesn't represent the rainwater of that time but 1000 years before. that changes our time frame and thus the different finding.

SANGEET77 · Answer

Hi,

Can anyone please explain option D . What does that mean ?

Why to eliminate option A ?

sangarajubharadwaj · Answer

Option A says "A) There is little deuterium in the North American caves than the deuterium in the water trapped there.  "This means other than the Deuterium that is available in the trapped rain water in the caves, there is not much (or no) deuterium from other sources.

Firstly, we want some explanation that tells us why the deuterium in caves is lesser than that in the trees, or why the deuterium in trees is more than that in caves. If we can find any reason to explain the above discrepancy, then that should be the correct answer. Option A doesn't say anything about it.

Option A says Deuterium content is lesser in caves, but the content whatever is present is totally from rainwater.  So it just eliminates any chance to further strengthen the PARADOX. But it does not give any reason to resolve the paradox.
 
So A can't be the answer. Please acknowledge my reply.

-Bharadwaj

Rajatverma17 · Answer

Situation

In North America, wood from trees that grew 16,000 to 24,000 years ago contains more
deuterium than wood from trees growing today. But water in caves that formed during that same
period contains less deuterium than rainwater contains today.

Reasoning

What could explain the puzzling discrepancy between the observed deuterium levels in wood and in
caves? Since the deuterium content of wood from trees reflects the deuterium content of rainwater
available to the trees while they grew, the deuterium levels observed in wood suggests that North
American rainwater contained more deuterium 16,000 to 24,000 years ago than it contains today.
But this conclusion seems at odds with the low deuterium levels in water in caves that formed
16,000 to 24,000 years ago. Several factors might explain the discrepancy: the water in those caves
might not be rainwater from the period when the caves formed; or some natural process might
have altered the deuterium levels in the cave water or the wood; or the wood or caves in which
deuterium levels were measured might be statistically abnormal somehow.

A: If the caves had absorbed deuterium out of the rainwater trapped in them, there would probably be
deuterium in the cave walls. So the observation that there is little deuterium in the caves apart from that in
the water eliminates one possible explanation for the oddly low deuterium levels in the cave water.

B: This suggests that the deuterium levels in the wood accurately reflect higher deuterium levels in rainwater
that fell 16,000 to 24,000 years ago, but it does not explain why the deuterium levels are so low in water in
the caves that formed then.

C: This could explain why deuterium levels in rainwater have changed, but it does not help explain the
discrepancy between the high deuterium levels in the wood and the low deuterium levels in the cave water.

D: Correct. If the water in the caves fell as rainwater thousands of years before the caves formed, it may date
from a period when rainwater contained much less deuterium than during the period 16,000 to 24,000
years ago, and much less than today.

E: If wood with high deuterium content were more likely to be preserved, then wood from 16,000 to 24,000
years ago might have a high deuterium content even if the rainwater then had a low deuterium content. So
the observation that wood with more deuterium is not more likely to be preserved eliminates one possible
explanation for the discrepancy.

The correct answer is D.

custodio · Answer

I found something strange.

From the pasage, we know that (DT as deuterium):
1- High/Low DT content of wood --> [b]High/Low DT content of rainwater (available to trees during their growth)
2- DT content of wood 16k-24k years ago > DT content of wood now

Here, we would expect that:
3- DT content of rainwater 16k-24k years ago > DT content of rainwater now

But, we found that
DT content of water 16k-24k years ago in cages < DT content of rainwater now
even though we would expect that
DT content of water 16k-24k years ago in cages > DT content of rainwater now

Th questions asks us why it is < instead of >.
*******************************************************
I understand choice (D) explains the paradox. However, choice (D) says that water trapped in the caves are from earlier than 16k-24k years ago. In order for (D) to be true,
D content of rainwater earlier than 16k-24k years ago < D content of rainwater earlier 16k-24k years ago.

Namely, timeline says:

Year:-----------------------------earlier than 16k-24k------------------16k-24k-----------------------Now
DT content of rainwater is:------too low (e.g. DT=5)-------high (e.g. DT=100)-------low (e.g. DT=10)

Any opinion?

Nonktp · Answer

GMATNinja  could you please explain why C is incorrect whereas D is correct

CrackverbalGMAT · Answer

Hi

Please allow me to try to clarify your question.

As per the stimulus, there is a paradox in the following:

Fact 1 : Wood from trees that grew between 16,000 and 24,000 years ago in North America contains significantly more deuterium than wood from trees growing today.
 Fact 2 : Water trapped in several North American caves that formed during that same early period contains significantly less deuterium than rainwater in North America today.

Therefore we have the paradox:

Deuterium in wood (16k-24k years) > Deuterium in wood from trees growing today (presumably =) Deuterium in rainwater in North America today > deuterium in water trapped in several North American caves that formed before 16k - 24k years.

We need to reconcile:

Deuterium in wood (16k-24k years) > Deuterium in water trapped in several North American caves that formed before 16k - 24k years

Now, option (C) states: Industrialization in North America over the past 100 years has altered the deuterium content of rain.

This merely tells us why Deuterium content in rain today is different (presumably lower) than Deuterium content in rain earlier. This does not explain how two instances that apparently drew rainwater from the same period (16k-24k years ago) have different Deuterium contents.

Option (D) addresses this by telling us that the caves could have sourced their wait from rain that fell thousands of years before the caves formed, and hence offers a potential explanation for the paradox.

Hope this helps.

GK002 · Answer

How in the world is this Q classified as low difficulty? I eliminated all the answer choices because I couldn't identify one that clearly explains the two differences.

Difference 1: Rainwater in OLD trees have  more  Deuterium than today's trees
Difference 2: Rainwater in OLD caves have  less  deuterium than rainwater today

How does D reconcile these two??

woohoo921 · Answer

I noticed that this problem uses "rainwater" and then "water". Is this a subtle hint that the water may not be the same (e.g., the water in the cave could theoretically be tap water and not rainwater, and would this be another example of a way to resolve the discrepancy?)

In addition, in my initial read of the question, I assumed that in general older trees have more deuterium, and I would have missed the correct answer choice based on this. How do you avoid making a mistake like this? It is tricky on CR because sometimes the argument is relying on an assumption like this vs. in this case reading carefully for the exact timeframe (16,000 to 24,000 years) matters.

ReedArnoldMPREP · Answer

I think that's fair. Would be curious other experts' thoughts on this though.

Anything that implied 'the water in the caves is different from rainwater that fell during that time' would be a possible explanation.

Not quite sure what you're saying your mistake was. Are you saying you turned 'wood from trees in that time period have more deuterium than trees do today' into 'trees from that period had more deuterium than trees do today?'

That is the kind of subtle thing that can cost points. E, for instance, is onto something, but it's in the negated form. If deuterium-heavy wood is more likely to be preserved, it's not that old trees actually had more deuterium than trees today--we're dealing with a classic survivor bias (something the GMAT tests with some regularity).

So, yes--be careful about snapping what's written into something similar but crucially different. 'wood   from that time have more deuterium than do trees today' is substantively different from 'trees from that time definitely had more deuterium than do trees today.'

GMATNinja · Answer

To start with your second point: even if an argument relies on an assumption, you don't want to just fill in that blank as if it's explicitly stated in the passage. If you do that, you're actually "fixing" the author's argument as you go, which will then make it impossible to accurately strengthen/weaken/etc. the argument that exists on the page.

Instead, just think about the structure of the argument as written, without filling in any gaps in the author's reasoning. That will prevent you from inserting your own assumptions as you did in this question.

Back to your first question: it's good to notice differences in language, and it's true that "water" can come from any source, whereas "rainwater" only comes from rain. But that in itself doesn't actually help us resolve the discrepancy.

Here's that discrepancy:

On one hand, there's evidence that the rainwater available to trees 16000-24000 years ago had MORE deuterium than rainwater today 
 On the other hand, the water in caves 16000-24000 years ago contains significantly LESS deuterium than rainwater in North America today.   
Could this discrepancy be resolved by info explaining that the water came from two different sources? Sure! But your job is not to think of your  own  answer choice to resolve the discrepancy -- instead, your job is to assess the options given to you.

(D) resolves the discrepancy in a completely different way: both the caves and the trees were impacted by rainwater, but the rainwater in the caves is much older than the rainwater drunk by the trees. This helps us reconcile the two findings, and so (D) is the correct answer.

I hope that helps!

woahsun · Answer

I know this is late but I agree. It's the SAME water.

DmitryFarberMPrep · Answer

On the contrary, D is establishing that it may NOT be the same water. The trees may have grown at the same time that the caves formed, but D is saying that the trees used CURRENT water, but the water in the caves may be older than the caves themselves. Therefore, the water that formed the trees and the water in the caves may be from different eras, so there's no mystery in their having different deuterium levels.

bhanu29 · Answer

(A) There is little deuterium in the North American caves other than the deuterium in the water trapped there.
No mention about the trees result, irrelevant.

(B) Exposure to water after a tree has died does not change the deuterium content of the wood.
Irrelevant.

(C) Industrialization in North America over the past 100 years has altered the deuterium content of rain.
Still doesn't seem to show why concentrations are different.

(D) Trees draw on shallow groundwater from rain that falls during their growth, whereas water trapped in caves may have fallen as rainwater thousands of years before the caves formed.
Bingo, informs us exactly why trees have more water as it absorbs water throughout its growth period, but caves have trapped water.

(E) Wood with high deuterium content is no more likely to remain preserved for long periods than is wood with a low deuterium content.
No mention about caves,

Rainwater contains hydrogen of a heavy form called deuterium. The

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GMAT Critical Reasoning (CR) Questions

Rainwater contains hydrogen of a heavy form called deuterium. The

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