I think it is important when you answer this question to make sure you realise how Frost uses fire and ice respectively as symbols. Of course, various scientists and other people have predicted that the world will end by being burnt to a crisp or by another ice age that will freeze us all up, but Frost uses fire and ice as symbols of desire and hatred respectively. Note how lines three and four end with "desire" and "fire" and then lines 6 and 7 send with "hate" and "ice." Thus Frost selects ice and fire as the two possibilities as he is commenting on various predictions about the end of the world and how they seem to focus on another ice age or an inferno, but then he goes on from this to make a larger point about the two emotions of desire and hatred.