OE
The inference that Antarctica was discovered by the ancients would be weakened if there were an alternative explanation of why the ancients might have drawn a landmass in that area on their maps. Choice E provides just such an explanation, so it is the correct answer.
Choice A is incorrect because the identity of the modern discoverer of the Antarctica has no bearing on why the continent was included on sixteenth-century maps. Since the ancients referred to in the passage likely postdate the warm period mentioned in Choice B, that option is also incorrect.
The passage never mentions how many sixteenth-century maps show a southern polar landmass, and the argument does not depend upon any particular quantity, so Choice C is incorrect.
Choice D comments upon the conclusion but does not pertain to the argument itself, so it is also incorrect.
Answer: E