Which theme is shown mostly clearly in this excerpt

When I look at the answer choices, choice A would make sense for the first sentence, but not the rest. Choice B is supported by the part of the passage that says,  "...I am weightless again, flying past the other runners". The choice C isn't supported whatsoever because the passage does not say whether or not Squeaky wins, nor does it talk about working hard to prepare for the race. The choice D is supported by the positive self-talk and the resulting physical response (running faster) that the passage describes. This is why I believe D is the best answer.

D. Success is possible if one truly believes in oneself.

i would say the correct answer is not concrete.