"those are the book's authors" (several authors of one book) or maybe "books' authors" (authors of some books, both "authors" and "books" plural)Dear students, these are my words, those are book's authors'
Number 2 is correct.
Number 3 should be "Which ones do you like best, my paintings or dad's?" "Mine" is used without a noun following it; if you wanted to use "mine" then the correct sentence would be: "Which ones do you like best, dad's paintings or mine?"
And about that pencil...
"Which pencil would you like to take to school, Peter's or Marisa's?" OR "Would you like to take to school Peter's pencil or Marisa's?" OR
"Would you like to take to school Peter's or Marisa's pencil?"
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