PT94.S1.Q18 (G4) - Help "If the collector purchases exactly two books, which one must be true"

mtakesLSAT

I think I understand why D is the correct answer choice. However, I am curious as to what you guys think is the best way to approach this question? Clearly using an in an out game board is the best choice. However, this question was hard for me to answer quickly just because it felt like a question that required a lot of trial and error, which obviously didn't work out (especially time wise).