Midterm Multiple Choice Questions

So this semester I faced a challenged on the midterm exam. Since every student was asked the same 20 questions on the chapter quizzes, how was I to select the 8 per chapter for the midterm? I started by selecting questions at random, but then I saw that some of them were a bit obscure and had been missed by a majority of students when they were asked on the chapter quiz. Instead I decided to ask the 8 that the students did best on from the chapter quizzes. That should help them do better on the midterm. I also made sure not to include any questions that referred to maps in the textbook.