Medical students compared two types of knee surgery to determine if patients were able to return to their active lifestyles. They studied partial knee replacement and total knee replacement. Identify the response and explanatory variables. The response variable is the type of knee surgery, and the explanatory variable is the amount of activity the patients were able to do after surgery. The response variable is total knee replacement, and the explanatory variable is partial knee replacement. The response variable is partial knee replacement, and the explanatory variable is the amount of activity the patients were able to do after surgery. The response variable is the amount of activity the patients were able to do after surgery, and the explanatory variable is the type of knee surgery. The response variable is partial knee replacement, and the explanatory variable is total knee replacement.