Critical Reasoning

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.

OA Later

I found this one in an LSAT test. I think it is pretty good. OA after some discussion...

IMO -- A

rkanthilal wrote: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.

OA Later

I found this one in an LSAT test. I think it is pretty good. OA after some discussion...

brown dw.jim stars are so cold to burn hydrogen and red stars are so hot to burn hydrogen.
All stars have an amount of lithium. ALL stars are hot enough to destrou lithium but the coolest brown stars=> the coolest brown drafts have lithium => the coolest brown drafts stars are not hot enoght to destroy lithium...
statement A is just the same
A is the answer....

THE INFORMATION
Red stars can burn Hydrogen.
Brown dwarfs cannot, therefore they are cooler than Red stars.
(this information is actually not relevant to the question.)

Only the coolest brown dwarfs cannot burn Lithium (all others can)

THE CLAIM
No Lithium => Not one of the coolest brown dwarfs

THE ASSUMPTION
Note that "only the coolest Brown dwarfs cannot burn Lithium" means these "coolest Brown dwarfs are not hot enough to burn Lithium NOW." What about earlier?

The assumption here is that Brown dwarfs were never hotter than they are now.

If it was possible for Brown dwarfs to be hotter earlier, then it would be possible that:
A Brown dwarf was hotter (earlier), and destroyed all the Lithium AT THAT TIME. Then, it cooled down and became one of the "coolest Brown dwarfs" NOW.

But, if the assumption is true, then this possibility is excluded.

Manit
GMAT trainer
[email protected]

You guys got it right. The OA is A.