Karla's AP® Psychology teacher wants to make sure students are reading every night. She tells them they will have quizzes, but does not tell them when. The first week, Karla had a "pop quiz" on Monday, but the next week she had a "pop quiz" on Wednesday and Thursday. This week, Karla is going to read every night because she wants an "A" and does not know when the next "pop quiz" will be. Karla's AP® Psychology teacher is using which schedule of reinforcement?