The two 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 phrase "who this letter is addressed to" is often debated regarding its correctness, as some grammarians argue that it 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 use of "and therefore" in this context is often critiqued, as some prefer to use "therefore" by itself or find the combination awkward.
These sentences exhibit elements of contested usage in terms of grammar and language conventions.