This is because the sound you make when you say x is ex thus using a but when you make the b sound it sounds like be.
Answer:Keats uses here two elaborated metaphors: one of the imagination as a charioteer who can fly into the heavens and "do strange deeds / Upon the clouds" (evidently a reference to the imagination's creative func tion), and one of poetry itself as being a planet of sound, rolling through the heavens.
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