The sentences that include errors in the use of possessives or contractions are:
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"We are not certain who's textbooks these are, but we want to find out so that we can retur them." (The correct word should be "whose" instead of "who's" since it indicates possession.)
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"Don't forget to pack your coat and hat, because its going to be snowing in Minnesota all week." (The correct word should be "it's" instead of "its" since it is a contraction for "it is.")