AbdurRakib wrote:
GMAT® Official Guide 2017Practice QuestionQuestion No.: CR 587
Page: 523
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.
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.
PrethinkingThought processWe 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 AnalysisOption AThe 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 BThis 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 CThis 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 DThis is in line with our pre-thinking.
Thus, this is the correct answer choice.
Option EThe 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.
_________________