A researcher wants to test whether casein, a protein found in milk causes cancer. To test this, the researcher uses rats. The rats are the same age, same sex, healthy, and have the same type of shelter and diet. The only difference is that one group of rats have casein in their food, and the other group of rats does not have casein in their food. After months of examining the effects of casein on cancer production, the researcher concludes that casein causes cancer in rats. Which of the following is a testable hypothesis for this experiment?