The most common treatment for breast cancer discovered in its early stages was once removal of the breast. Today, the usual practice is to remove only the tumor and nearby lymph nodes, with the surgery then being followed by radiation. To study whether these treatments differ in their effectiveness, a medical team examines the records of 25 large hospitals and compares the survival times after surgery of all women who have had either treatment. (a) The explanatory variable is O the removal of the breast. O the treatment that the woman has had. the survival times after surgery. the medical team. The response variable is the survival times after surgery. the medical team. Othe removal of the breast.