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I'd say that the statement from FDR's speeches that uses parallel structure is C. American ships have been sunk; American airplanes have been destroyed.
Both of these independent clauses use the passive form of the verb - in the first clause, the passive form of the verb <em>to sink </em>is used, and in the second, of the verb <em>to destroy.
</em>A parallel structure means that throughout a sentence, the same form of words is used, and given that here both sentences have passive forms, I'd say C is the correct answer.<em>
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