This stamp is probably highly valuable, since it exhibits a printing error. The most important factors in determining a stamp' s value, assuming it is in good condition, are its rarity and age. This is clearly a fine specimen, and it is quite old as well. The conclusion is properly inferred if which one of the following is assumed? ['Most stamps with printing errors are already in the hands of collectors.', "Rarity and age are of equal importance to a stamp's value.", 'Printing errors are always confined to a few individual stamps.', 'Even old and rare stamps are usually not valuable if they are in poor condition.']