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

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

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05 Feb 2007, 13:09
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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 cloudsâ€™ 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 inferences about an individual rain cloud is supported by the passage?

(A) Once it is formed over the Atlantic, the rain cloud contains more ordinary oxygen than oxygen-18.
(B) Once it has passed over the Amazon, the rain cloud contains a greater-than-normal percentage of oxygen-18.
(C) The clouds rainfall contains more oxygen-18 than ordinary oxygen.
(D) During a rainfall, the cloud must surrender the same percentage of its ordinary oxygen as of its oxygen-18.
(E) During a rainfall, the cloud must surrender more of its oxygen-l8 than it retains

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SVP
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Re: CR - Oxygen 18 [#permalink]

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05 Feb 2007, 13:42
SimaQ wrote:
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 cloudsâ€™ 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 inferences about an individual rain cloud is supported by the passage?

(A) Once it is formed over the Atlantic, the rain cloud contains more ordinary oxygen than oxygen-18.
(B) Once it has passed over the Amazon, the rain cloud contains a greater-than-normal percentage of oxygen-18.
(C) The clouds rainfall contains more oxygen-18 than ordinary oxygen.
(D) During a rainfall, the cloud must surrender the same percentage of its ordinary oxygen as of its oxygen-18.
(E) During a rainfall, the cloud must surrender more of its oxygen-l8 than it retains

This is a twisted 'resolve the paradox' question.

D. For the content to be constant, the ratio of the proportion of O2-18 surrendered and the proportion of O2-normal surrendered should be the same.

But in rainfall, a higher proportion of all water molecules containing oxygen-18 than of all water molecules containing ordinary oxygen descends to earth.

This does not mean that a higher proportion of O2-18 is surrendered, it only means that a higher proportion descends on the earth. There is a possibility that the majority of normal O2 surrendered do not descend for some unknown reason.

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VP
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05 Feb 2007, 13:44
I go with D, the clouds are losing oxygen 18 , however if the proportion of the oxygen is to remain constant, the passage is inferring that normal oxygen must be leaving in some other way as well

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Senior Manager
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Re: CR - Oxygen 18 [#permalink]

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05 Feb 2007, 15:15
D.

In these questions have to be careful when picking between numbers and percentages.

D says that to keep levels of Oxygen 18 even, after it rains, the cloud must also lose regular oxygen to maintain the ratio.

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Director
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06 Feb 2007, 00:53
However D is not the OA....

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Senior Manager
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06 Feb 2007, 08:55
SimaQ wrote:
However D is not the OA....

Is it C?

the higher proportion of rain is oxygen 18, therefore the rain content has more oxygen 18 than normal oxygen.

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Director
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07 Feb 2007, 01:11
OA is A... which is not correct i think.

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07 Feb 2007, 08:43
SimaQ wrote:
OA is A... which is not correct i think.

I guess I can see how A is supported, but so are a lot of the answer choices.

I have trouble with CR and RC inference questions. Anybody have a clue on this one? There seems to be a lack of consensus on an official or unofficial OA.

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Director
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07 Feb 2007, 09:07
Clouds have more O16 > O18
But rainfalls contain higher proportion of O-18 than normal O16
IMO, which means clouds release more O18 than O16.

B. Wrong.
C. Opposite is true.
D. Canâ€™t be true because the clouds have more of O18 than O16, so they have to release more of their O18 in order for the rainfalls to have O18 > O16
E. We know nothing about retention.

By POE A.

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Senior Manager
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07 Feb 2007, 09:28
asaf wrote:
Clouds have more O16 > O18
But rainfalls contain higher proportion of O-18 than normal O16
IMO, which means clouds release more O18 than O16.

B. Wrong.
C. Opposite is true.
D. Canâ€™t be true because the clouds have more of O18 than O16, so they have to release more of their O18 in order for the rainfalls to have O18 > O16
E. We know nothing about retention.

By POE A.

asaf, where are you getting O16 from?

Am I missing something? How do you refute C?

Stem says:
But in rainfall, a higher proportion of all water molecules containing oxygen-18 than of all water molecules containing ordinary oxygen descends to earth.

Therefore, we can infer C:
The clouds rainfall contains more oxygen-18 than ordinary oxygen.

Any insights are appreciated.

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Director
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07 Feb 2007, 10:32
buckkitty,
Above, I used O16 for normal oxygen and O18 for its heavier version.

At first I read C differently but now that I read it again, I may have a different explanation to offer!

In C, I am not sure, what is â€˜clouds rainfallâ€™? Ok, even, if we assume if it is a simple rainfall than this is not really an inference as it is explicitly mentioned.

While, nothing what is provided in â€˜Aâ€™ is explicitly provided but we construe that from the â€˜clouds maintaining constant O18â€™ part.

I, again, will pick A.

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Manager
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19 Feb 2007, 13:16
'A'

If cloud contans more oxygen 18, it would rain. It does not happen at the time of cloud formation on Atalantic but happens on Amazon, where the same cloud contains more oxygen 18 than normal oxygen.

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19 Feb 2007, 13:16
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# Oxygen-18 is a heavier-than-normal isotope of oxygen. In a

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