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? ['Most stars that are too cool to burn hydrogen are too cool to destroy lithium completely.', 'No stars are more similar in appearance to red dwarfs than are brown dwarfs.', 'Most stars, when first formed, contain roughly the same percentage of lithium.', 'None of the coolest brown dwarfs has ever been hot enough to destroy lithium.']