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OG CR:Rainwater contains hydrogen of a heavy

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NandishSS Master | Next Rank: 500 Posts Default Avatar
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OG CR:Rainwater contains hydrogen of a heavy

Post Fri Sep 16, 2016 11:00 pm
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 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.

OA:D

Source:OG 2017

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crackverbal Master | Next Rank: 500 Posts
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Post Tue Mar 07, 2017 4:15 am
this is what we know -

1. Wood from trees between 16,000 to 24,000 years ago contains significantly more deuterium than wood from today. Hence, it contains more deuterium than rainwater today (because deuterium content of wood reflects the deuterium content of rainwater).
2. Water trapped in North American caves that formed during the same period contains significantly less deuterium than rainwater in North America today.

We have been asked to explain why water trapped in North American caves has such a low deuterium content.

A - Incorrect.
"less deuterium in the the caves than in the water trapped there" - does not reconcile points #1 and #2.

B - Incorrect.
does not explain why the deuterium content in the water trapped in caves is lower.

C - Incorrect.
we are not concerned about deuterium content 100 years ago but with deuterium content 16,000 - 24,000 years ago.

D - Correct.
this implies that water trapped in caves is not from the same period - 16,000 to 24,000 years ago. So, we cannot make appropriate comparisons about deuterium content.

E - Incorrect.
does not explain why water trapped in caves has lower deuterium content.
If it was more likely to be preserved, then that would explain the high deuterium content in it.

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NandishSS Master | Next Rank: 500 Posts Default Avatar
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Post Tue Feb 28, 2017 5:53 pm
Bumping the thread for the experts review.

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