Oxygen-18 is a heavier-than-normal isotope of oxygen. In a : GMAT Critical Reasoning (CR)
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# Oxygen-18 is a heavier-than-normal isotope of oxygen. In a

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Oxygen-18 is a heavier-than-normal isotope of oxygen. In a [#permalink]

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30 May 2009, 23:34
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15. Oxygen-18 is a heavier-than-normal isotope of oxygen. In a rain cloud, water molecules containing oxygen-18 are rarer than water molecules containing normal oxygen. But in rainfall, a higher proportion of all water molecules containing oxygen-18 than of all water molecules containing ordinary oxygen descends to earth. Consequently, scientists were surprised when measurements along the entire route of rain clouds’ passage from above the Atlantic Ocean, the site of their original formation, across the Amazon forests, where it rains almost daily, showed that the oxygen-18 content of each of the clouds remained fairly constant.

Which one of the following statements, if true, best helps to resolve the conflict between scientists’ expectations, based on the known behavior of oxygen-18, and the result of their measurements of the rain clouds’ oxygen-18 content?
(A) Rain clouds above tropical forests are poorer in oxygen-18 than rain clouds above unforested regions.
(B) Like the oceans, tropical rain forests can create or replenish rain clouds in the atmosphere above them.
(C) The amount of rainfall over the Amazon rain forests is exactly the same as the amount of rain originally collected in the clouds formed above the Atlantic Ocean.
(D) The amount of rain recycled back into the atmosphere from the leaves of forest vegetation is exactly the same as the amount of rain in river runoffs that is not recycled into the atmosphere.
(E) Oxygen-18 is not a good indicator of the effect of tropical rain forests on the atmosphere above them.

OA is
[Reveal] Spoiler:
b
.

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31 May 2009, 03:00
OA is C

(A) Rain clouds above tropical forests are poorer in oxygen-18 than rain clouds above unforested regions. ---This is irrelevant since this does not address the reason for constant rainfal in Amazon forest
(B) Like the oceans, tropical rain forests can create or replenish rain clouds in the atmosphere above them.---study in question is the formation of cloud over Atlanic Ocean and constant rain fall in Amazon.So this is ruled out
(C) The amount of rainfall over the Amazon rain forests is exactly the same as the amount of rain originally collected in the clouds formed above the Atlantic Ocean.--This clealy validate the rlationship between formation of clouds and constant rainfall
(D) The amount of rain recycled back into the atmosphere from the leaves of forest vegetation is exactly the same as the amount of rain in river runoffs that is not recycled into the atmosphere.---This is not on the context of the topic
(E) Oxygen-18 is not a good indicator of the effect of tropical rain forests on the atmosphere above them.--This is also out of context of the topic
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01 Jun 2009, 08:04
Is the OA B or C?
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01 Jun 2009, 11:46
vaivish1723 wrote:
Consequently, scientists were surprised when measurements along the entire route of rain clouds’ passage from above the Atlantic Ocean, the site of their original formation, across the Amazon forests, where it rains almost daily, showed that the oxygen-18 content of each of the clouds remained fairly constant.

We need to look for an option which explains why the oxygen-18 content of each of the clouds remained fairly constant.

vaivish1723 wrote:
(B) Like the oceans, tropical rain forests can create or replenish rain clouds in the atmosphere above them.

B does it very nicely.
Tropical rain forests create or replenish rain clouds in the atmosphere above them.And thus keep O-18 content constant even if it rains daily in Tropical rain forests.
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03 Jun 2009, 10:07
i still dint get it ..

Can anyone care to explain it in more detail .. ?
Re: Oxyger isotope   [#permalink] 03 Jun 2009, 10:07
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