Let's see if we can help you figure the answer out for yourself so you'll remember it better. What exactly do we mean by cooperative binding when talking about oxygen and hemoglobin? How do we know it happens (i.e., what is measured)?
You should be able to answer the first of my two questions. The second may depend on how you covered it in class. If you don't know the second part, don't worry, we can work through that so you'll know even more than the rest of your class.