Imagine that the university of minnesota introduces a radically new kind of grading system. Now, when you take a class, every minute that you spend reading and preparing for that class can be recorded accurately. For each ten minutes you spend studying, you earn a point. You can accumulate as many points as you want just by spending more time studying for class. At the end of the semester, your grade will be based on how much time you have spent studying. This radical grading system is based on what kind of reinforcement schedule?.