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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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16 Apr 2008, 11:55
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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 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 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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16 Apr 2008, 12:46
C ?
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16 Apr 2008, 12:55

Usually the clouds have less of Oxygen 18. Rainfall is heavy in O18 so after rainfall in Amazon the clouds' O18 is further depleted. Since scientists have found that the oxygen-18 content of each of the clouds remained fairly constant, the O18 must have been replnished.
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16 Apr 2008, 17:36
C is the only one that can be conclusively drawn from the passage.

saravalli 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 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 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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17 Apr 2008, 11:08
A? the stem states that in a rain cloud, oxygen 18 are rarer than normal oxygen aka: rain cloud contains more ordinary oxygen than oxygen 18??
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17 Apr 2008, 11:36
I go with B.
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17 Apr 2008, 11:49
OA says A. any expln is appreciated.
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18 Apr 2008, 11:11
I would still like to go for C. Reason question says...Which one of the following inferences about an individual rain cloud is supported by the passage?
A says about the particular rain cloud quoted as example which forms at the site of atlantic.

Although A and C both can be infered from question. But C is more general inference and A is the inference just for the particular rain cloud that has been quoted in the passage as an example. So if I have to chose inference about an individual rain cloud then I would go for C.

Any other explanations are more than welcome.
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18 Apr 2008, 11:45
A is the correct answer. My explanation below. Hope it helps!

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.

To sum what passage is trying to say:
- rain cloud has lower percentage of oxygen-18.
- however, rainfall has higher percentage of oxygen-18
- when cloud moved from Atlantic to Amazon, the content of oxygen-18 in clouds remained same.
- scientist were suprised by the above observation (may be because they expected it to not be same).

For inference, no answer choice be outside what is stated in the passage. So let us examine each.

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. correct. It is correctly reflected in the second sentence of the passage above.
(B) Once it has passed over the Amazon, the rain cloud contains a greater-than-normal percentage of oxygen-18.Incorrect- passage never mentions about what happens to cloud after is passes over Amazon. It only mentions between Atlantic and Amazon.
(C) The clouds rainfall contains more oxygen-18 than ordinary oxygen..What is a cloud rainfall? This term is ambiguous and not mentioned anywhere in the passage.
(D) During a rainfall, the cloud must surrender the same percentage of its ordinary oxygen as of its oxygen-18. .Incorrect. This is not stated in the passage?
(E) During a rainfall, the cloud must surrender more of its oxygen-l8 than it retains. .Incorrect. This is no where mentioned in the passage.
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22 Apr 2008, 13:35
Thanks for the discussion.
Re: CR-Oxygen2   [#permalink] 22 Apr 2008, 13:35
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