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

Question

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.

CrackverbalGMAT · Accepted Answer

Pre-thinking:

We are told the following:

In a cloud

Let proportion of water molecules containing Oxygen-18 (hereafter, O-18) = x
Let proportion of water molecules containing normal Oxygen (hereafter, normal-O) = y = 1-x
y>x

During rainfall, some water molecules among x (say, a%) and some water molecules among y (say, b%) fall to the earth.

Now, a>b. So if a greater proportion of water molecules containing O-18 fall to the earth than those containing normal-O, we would expect the proportion of water molecules containing O-18 to steadily decrease over time ie; we would expect x to steadily decrease. However, measurement of clouds across the Amazon, where it rains almost daily, showed x to be constant.

Initially I thought this would be a resolve the paradox question. However, it turned out to be a much simpler inference based question!

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.  This states y>x, which is true. However, I am not convinced that this is an "inference" as such, because this is quite explicitly stated in the question. At this point, we can hold on to this option. 
 
(B) Once it has passed over the Amazon, the rain cloud contains a greater-than-normal percentage of oxygen-18.  Incorrect. The passage clearly states "...the oxygen-18 content of each of the clouds remained fairly constant". Additionally, we do not know what is considered a "normal" percentage of O-18.

(C) The clouds rainfall contains more oxygen-18 than ordinary oxygen.  Incorrect. We only know that a>b, which are the respective  proportions  of water molecules containing O-18 and normal-O. Whether the absolute quantity of O-18 in rain is greater than normal-O, we cannot say.

(D) During a rainfall, the cloud must surrender the same percentage of its ordinary oxygen as of its oxygen-18.  Incorrect. The passage clearly states that a>b.

(E) During a rainfall, the cloud must surrender more of its oxygen-l8 than it retains.  Incorrect. This states that a>1-a. We cannot know this for sure.

Given that all other options are so obviously contradictory to the facts stated in the passage, we can select option (A).

Hope this helps.

creed4truth · Answer

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.

This is what I think,
As per question:
Rain Cloud O-18 < Rain Cloud O
Rain Fall:
Rain Fall O-18/ Total O-18 >Rainfall O/Total O

Rain Cloud O-18 content remains fairly constant while crossing Amazon where Rainfall happens daily.

I think this is only possible if Rain Cloud O is a pretty big number
e.g:
Rain Cloud O-18 = 10
Rain Cloud O=10000

so Rain Cloud O-18< Rain Cloud O true
Daily Rainfall in Amazon:
Rainfall O-18 =1/10
Rainfall O = 99/10000
so Rainfall O-18/Total O-18 > Rainfall O/Total O

I feel that satisfies the fact that despite Heavy Rain Fall the RainCloud O-18 remains fairly constant cause Rain Cloud O contributes primarily to the incessant rainfall.

(B) Once it has passed over the Amazon, the rain cloud contains a greater-than-normal percentage of oxygen-18.Not sure about this, but it tells us that the clouds contain a greater than normal % of O-18? What is the normal percent?-Eliminated
(C) The clouds rainfall contains more oxygen-18 than ordinary oxygen. We are concerned about the Cloud so it's Out of Scope
(D) During a rainfall, the cloud must surrender the same percentage of its ordinary oxygen as of its oxygen-18.in the text above, we know that O-18 in rainfall is more than O, but nothing is mentioned about the percentage these values represent of their content in the cloud itself
(E) During a rainfall, the cloud must surrender more of its oxygen-l8 than it retains.Out of scope, Rainfall does contain more O-18 but nothing is mentioned about how much is retained or what fraction is retained.

unraveled · Answer

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. - CORRECT. Already stated in passage though not explicitly but again this is how it is.
(B) Once it has passed over the Amazon, the rain cloud contains a  greater-than-normal percentage of oxygen-18 . - WRONG. Whether greater or same or less, we are not sure if it's true. All three possibilities are equally likely or may be(all) not. 
(C) The  clouds rainfall   contains more oxygen-18 than ordinary oxygen . - WRONG. "Rain cloud" not "rainfall" is what we are looking for. Latter part is still considerable.
(D) During a rainfall, the cloud must surrender the  same percentage of its ordinary oxygen as of its oxygen-18 . - WRONG. Not sure about same percentage of molecules.
(E)  During a rainfall, the cloud must surrender more  of its oxygen-l8  than it retains . - WRONG. Problem is with retain part which we are not sure about. Thus, we can't infer.

Used POE to get the right answer but sadly in 4+ minutes. Took time mostly for the reason that options are confusing except the right one. So, a tough question the answer of which seems to easy to believe.
Take note of what is asked in question stem. It's about rain cloud and not rainfall. So, anything about rainfall is out of scope or irrelevant, however, in most likely terms and not in absolute terms.

Answer A.

Ilanchezhiyan · Answer

My take on this:
 Premise: 
O18 heavier than O2
Cloud---------------vs---------------- Rainfall
O2 Content > O18------------ O18 Content > O2;  
My inference is= as long as the water molecules remain as cloud nothing changes i.e  O2 Content > O18  ; But when it rains the O-18 begins to drop more than the O-2;
Therefore, A is correct IMO because it sticks to the original premise, that clouds have more O2 than O18;

Divyanshu14 · Answer

Bunuel , Can you please share the official answer?

DachauerDon · Answer

Lets identify the facts.
1: Oxygen 18 isotopes are heavier than normal Oxygen isotopes
2. In an cloud: water molecules with O18 < water molecules with normal O
3. During rainfall: quantity of water molecules with O18 that fall > quantity of water molecules without O18 that fall
4. O18 quantity remained constant in clouds formed over the Atlantic and travelling over the Amazon

Choice A) Facts 2 and 4 combined tell us that before rainfall occurs, there are less water molecules with O18 in clouds that formed over the Atlantic. Is it inferable that therefore, once it is formed over the Atlantic, the rain cloud contains more ordinary oxygen than oxygen-18? Yes. Do any of the other facts suggest that this inference might only be true sometimes and not always? No, they just provide more info about the weight of the molecules and what occurs during rainfall. Therefore, A is inferable.

The other answer choices all confuse percentages/ratios with quantities and have been explained very well by everyone else.

Note: I am no pro tutor and yet to take the exam but I like to provide my strategy/thinking when it worked well.

Vibhatu · Answer

"I am not convinced that this is an "inference" as such, because this is quite explicitly stated in the question. At this point, we can hold on to this option." if something is stated in the statement that can not be true ? if some other option is presented, do we need to choose that option? Yes someone obviously doesn't want to put so easy statement, but these does not mean interference question can not be stated clearly, in my opinion. Please share your thought.

Phani442 · Answer

I think you are comparing numbers with proportions based on which you have eliminated option C. The right explanation would be to state that it is already given in the premise that the number of O18 is lesser than number of Normal oxygen molecules. Correct me if I am wrong.
Thank you.

literaryzeitgeist · Answer

I do not know why this was tagged as a difficult question. Anyway, let us approach this in a straightforward manner. There are some facts that the argument lays out:

1. Oxygen-18 is a heavier-than-normal isotope of oxygen. 
2. In a rain cloud, water molecules containing oxygen-18 are rarer than water molecules containing normal oxygen.

Then comes a contradictory fact:

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

With #3 as the premise, we have:

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.

The surprise element is understandable because a higher proportion of all water molecules containing oxygen-18 than of all water molecules containing ordinary oxygen descending to earth when water molecules containing oxygen-18 are rarer than water molecules containing normal oxygen in a rain cloud is know would definitely be surprising if measurements along the entire route of rain clouds’ passage from above the Atlantic Ocean showed that the oxygen-18 content of each of the clouds  remained fairly constant .

The question asks us which of the given inferences about an individual rain cloud is supported by the passage. So let us read the options one by one:

(A) Once it is formed over the Atlantic, the rain cloud contains more ordinary oxygen than oxygen-18. - Supported by (2)
(B) Once it has passed over the Amazon, the rain cloud contains a greater-than-normal percentage of oxygen-18. - How do we know this? There is nothing in the argument that indicates this. In fact, one might argue it is the other way around. ELIMINATE
(C) The clouds rainfall contains more oxygen-18 than ordinary oxygen. - How do we infer this? The argument says more oxygen-18 than of all water molecules containing ordinary oxygen descends to earth. That does not imply the clouds rainfall itself contains more oxygen-18 than ordinary oxygen. - ELIMINATE
(D) During a rainfall, the cloud must surrender the same percentage of its ordinary oxygen as of its oxygen-18. - must surrender? Same percentage? How? Why? Seriously? - ELIMINATE
(E) During a rainfall, the cloud must surrender more of its oxygen-l8 than it retains. - How is this supported? - ELIMINATE

Answer is (A)

Feel free to share your thoughts! Any agreements or disagreements here?

B Sudharsan 
Founder of the  Literary Zeitgeist

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

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

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