The answer choice that best describes the tone of this passage is A. The tone is frustrated because It is very expensive to race in the Iditarod.
This refers to the author's outlook in a book that shows and sets the mood of the text.
Hence, we can see that the way the idiom selling your soul contributes to the meaning of this passage is that it shows that the mushers don't like having to rely on sponsors so they can race in the Iditarod
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