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The answer to this question is -could outrun. I hope this helps!
The answer to your question would be that the word or words modified by the underlined adverbial clause are the following ones: could outrun.
Adverb clauses, also known as adverbial clauses, are dependent clauses that function as adverbs. As they are dependent clauses, they require a subordinating conjunction to connect them to the rest of the sentence.