This question refers you to the conclusion sentence. The author states clearly that swimming must either progress into a new form or regress to the primal mode of movement. Choice B and D can be eliminated because they show no change, just a continuation of current circumstances. Choice A is an actual possibility as humans may very well be settling in outer space in the future. Choice C refers to animals anyway, when the author specifically was predicting a future for humans. Choice A is the most realistic and relevant prophecy according to the premises developed by the author.