The sentences that contain an example of contested usage are:
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"The postman needs to know who this letter is addressed to so it can be delivered." (The contested usage here is related to the phrase "who this letter is addressed to," which some grammarians argue should be "to whom this letter is addressed.")
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"Stephanie started reading the book first, and therefore she finished it before I did." (The contested usage here is the phrase "and therefore," where some argue it may not be necessary or could be contested whether the construction is the best way to convey the idea).
Please note, the repetition of sentences indicates potential confusion, but I addressed them based on the content provided. If any sentence looks different in each pair but they repeat exactly, they carry the same structure.