Answer :
C. To fly is the infinitive because an infinitive are two words which is to and another word like (to fly). If you see a three words and one of those words are to it is not an infinitive. (Example: To go there) That is NOT an infinitive. Hope I helped! :)
If i am not mistaken, it is C. I think this because the words(To fly), help continue the sentence. I also know because any infinitive starts with the word, "to." For example: To help; To want something; To fly. In other words, any word with to + the verb is an infinitive.
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