The sentences that include errors in the use of possessives or contractions are:
-
"We are not certain who's textbooks these are, but we want to find out so that we can return them."
- The error is in the use of "who's," which should be "whose" to indicate 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 error is in the use of "its," which should be "it's" (the contraction for "it is").
Thus, the correct answers are the first and the fourth sentences.