Brown dwarfs—dim red stars that are too cool to burn hydrogen—are very

Question

Brown dwarfs—dim red stars that are too cool to burn hydrogen—are very similar in appearance to red dwarf stars, which are just hot enough to burn hydrogen. Stars, when first formed, contain substantial amounts of the element lithium. All stars but the coolest of the brown dwarfs are hot enough to destroy lithium completely by converting it to helium. Accordingly, any star found that contains no lithium is not one of these coolest brown dwarfs.

The argument depends on assuming which one of the following?

(A) None of the coolest brown dwarfs has ever been hot enough to destroy lithium.

(B) Most stars that are too cool to burn hydrogen are too cool to destroy lithium completely.

(C) Brown dwarfs that are not hot enough to destroy lithium are hot enough to destroy helium.

(D) Most stars, when first formed, contain roughly the same percentage of lithium.

(E) No stars are more similar in appearance to red dwarfs than are brown dwarfs.

dj · Answer

A ~ 1.5 mins
it might have happened that a hot star converted into a cold (brown), but before that destroyed all the lithium. Now, you see it (earlier red) as a cold brown star. If this is false, the argument is true.

amit2k9 · Answer

hot ---> burn lithium 
not hot -##-> not burn lithium.

This logic is being exploited by A,the negation logic.

nachobioteck · Answer

This was quite easy for me (probably because I am a scientist).

Statement
-Brown dwarves cant destroy lithium

Conclusion
- If a star doesn't have lithium it is not a brown dwarves.

Posssible assumptions for this conc:

1- There are no stars that neve had lithium. If there were stars that never had lithium this could include brown dwarves.

2- Brown stars were never hot enough to destroy lithium. If at some point there were able to destroy then you could find brown dwarves that contain no lithium. This one matches one of our choices (A) is the correct answer

arirux92 · Answer

A : 1 minute

For reading the CR argument, time should not be a constraint!!!!

I can never stress enough how important it is to understand the argument. Every CR option is a trap and if you don't understand the argument and find the answer before you look at the options, I can assure you have a very high probability of getting the option wrong. ( Don't think of a clown with a red nose riding a blue bicycle.... You see what I did there?)

Coming to the question :

Premise : All stars contain lithium. But high temperature in stars causes li to be converted. Only the coolest of brown dwarf has temp low enough to sustain li.

Assumption: All stars contain li in the beginning. So do they directly go to brown dwarf phase? If not wouldn't li have been destroyed before it got there?

A is the clear winner

LogicGuru1 · Answer

Answer is A
To understand this question one must remember that stars are born ! 
Yes just like pop stars, rock stars, sports stars, film stars, The stars in sky are also born.
If you know this fact answer will come to you in 5 seconds. 
Stars are NAMED depending upon how much hydrogen they have burned and how much stronger their gravitational field:- some of the stars are RED GIANT, WHITE DWARF, BLACK HOLE, NOVA, SUPERNOVA, ETC ETC

Coming back to the question 
This is an assumption question. 
Easiest way to find answer is to isolate the CONCLUSION and then Negate the answer choices one by one.
The answer option which when negated makes the conclusion fail is the answer

OK
So now lets get to the task

Premise 1) All stars when born contains lot of Lithium
Premise 2) All stars burn Lithium to Helium 
Premise 3) Brown stars cannot burn Lithium to Helium
  Conclusion) Any star that contains no lithium is not a brown dwarfs.
Simplified Conclusion using contraposition ) Brown star contains lot of Lithium because they cannot burn lithium. (so all their lithium must still remain inside them)
 
  We have to attack this line "Brown star contains lot of Lithium because they cannot burn lithium."
 
What does answer choice A says
(A) None of the coolest brown dwarfs has ever been hot enough to destroy lithium.

Now when negated Answer choice A it becomes

"Some of the brown dwarf stars were once hot enough to burn Lithium"
 
 Now if brown stars at some point in their time burned all their lithium, they won't have lithium in them anymore. But according to conclusion there must be lithium in brown stars. So the conclusion fall apart because we can see that the star is brown star but contains no lithium.

hence A is the right answer.

Brown dwarfs' dim red stars that are too cool to burn hydrogen's are very similar in appearance to red dwarf stars, which are just hot enough to burn hydrogen. Stars, when first formed, contain substantial amounts of the element lithium. All stars but the coolest of the brown dwarfs are hot enough to destroy lithium completely by
converting it to helium. Accordingly, any star found that contains no lithium is not one of these coolest brown dwarfs.

The argument depends on assuming which one of the following?

(A) None of the coolest brown dwarfs has ever been hot enough to destroy lithium.
(B) Most stars that are too cool to burn hydrogen are too cool to destroy lithium completely.
(C) Brown dwarfs that are not hot enough to destroy lithium are hot enough to destroy helium.
(D) Most stars, when first formed, contain roughly the same percentage of lithium.
(E) No stars are more similar in appearance to red dwarfs than are brown dwarfs.

rvgmat12 · Answer

Source: Powerscore Forum

The stimulus begins by contrasting similar stars-- brown dwarfs and red dwarfs. Brown dwarfs are too cool, whereas red dwarfs are just hot enough, to burn oxygen. That particular contrast is actually irrelevant to the argumentation, which takes place after that first sentence. Since all stars upon formation contain lithium, and all stars except the coolest brown dwarfs can destroy lithium completely, any star that lacks helium is not one of the coolest brown dwarfs.

The argument doesn't make sense, because stars do not necessarily stay the same over their histories. For instance, there is no reason given to suppose that one of the hottest brown dwarfs couldn't destroy its lithium and then cool down later, thus becoming one of the coolest brown dwarfs, while lacking lithium.

You are asked on which assumption the argument depends, so you should focus on that unwarranted assumption.

Answer choice (A): This is the correct answer choice. The argument must assume that those coolest brown dwarfs have not, in the past, been hot enough to destroy their lithium.

Answer choice (B): Importantly, this incorrect choice treats the first sentence as if it plays a part in the argument. Since the first sentence merely supplies extraneous information, it is not connected to the main line of reasoning, and you should not believe that any connection is necessary. Also, since the stimulus makes conclusions about certainty, assumptions about majorities may not be helpful, and in any case this choice supports a conclusion about most brown dwarfs, but the stimulus only made a conclusion about the coolest brown dwarfs.

Answer choice (C): Since the stimulus makes no conclusion about the destruction of helium, this choice is irrelevant and incorrect. You should not have assumed that since helium is the result of the destruction of lithium, the stimulus claims that the helium persists. A result can in turn be destroyed, and the actual fact that helium requires a pretty high incineration temperature is irrelevant.

Answer choice (D): The stimulus states that stars are born containing substantial amounts of lithium, and it is in no way necessary to assume that they contain roughly equal percentages. You should not assume that detection capability is relevant to the argument, and this choice is wrong. You need to differentiate between the structure of an argument and the building blocks and results of the argument. When criticizing an argument or finding assumptions, you need to be interested in whether the premises presented a complete picture, and whether the premises definitely lead to the conclusion. You are not actually interested in whether the premises and conclusion are correct.

Answer choice (E): Since the comparison between brown and red dwarfs was utterly irrelevant to the main line of reasoning, this incorrect choice cannot represent a necessary assumption.

VerbalBot · Answer

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Brown dwarfs—dim red stars that are too cool to burn hydrogen—are very

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