Question ID:PT20 S1 Q16 Passage: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. Stem:The argument depends on assuming which one of the following? Correct Answer Choice:AChoice A:None of the coolest brown dwarfs has ever been hot enough to destroy lithium. Choice B:Most stars that are too cool to burn hydrogen are too cool to destroy lithium completely. Choice C:Brown dwarfs that are not hot enough to destroy lithium are hot enough to destroy helium. Choice D:Most stars, when first formed, contain roughly the same percentage of lithium. Choice E:No stars are more similar in appearance to red dwarfs than are brown dwarfs.