QUESTION TEXT: Brown dwarfs–dim red stars that are too…
QUESTION TYPE: Necessary Assumption
CONCLUSION: Any star without lithium isn’t a cool brown dwarf. [or: cool brown dwarfs have lithium.]
REASONING: Cool brown dwarfs aren’t hot enough to burn lithium. All stars have lithium when first formed.
ANALYSIS: The argument implies that some warmer brown dwarfs can burn lithium.
It’s possible that cool brown dwarfs were once warmer; warm enough to have burned all of their lithium.
By the time became cool brown dwarfs, they could have no lithium left.
The argument assumes that cool brown dwarfs were never warm enough to burn lithium.
___________
- CORRECT. If this isn’t true then cool brown dwarfs were once warmer and could have burned through their lithium.
- “Most” almost never is the right answer on necessary assumptions. This phrase could mean 51%, while the negation would be 50%. That can’t hurt the argument, which is making an absolute statement about all brown dwarf stars.
The answer choice is also irrelevant. It tells us that red dwarf stars are hot enough to burn hydrogen, but that doesn’t fit in with any of the information about lithium.
- We don’t know whether helium has anything to do with lithium. The part of the argument about helium is just there to mislead you.
- The argument should still be fine if stars contain differing percentages of lithium. The argument is assuming that cool brown dwarfs were never warm enough to burn it.
- Appearance is irrelevant. The first sentence was just fluff. We care about stars’ heat.
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