Researcher: We have found that some cases of high blood pressure can be treated effectively with medicine. Since it is generally accepted that any illness caused by stress is treatable only by the reduction of stress, some cases of high blood pressure must not be caused by stress. Which one of the following is an assumption required by the researcher's argument? ['Some conditions that are treated effectively by medicines are not also treatable through the reduction of stress.', "The reduction of stress in a person's life can at times lower that person's blood pressure.", "Reduced stress does not reduce a person's responsiveness to medicine used to treat high blood pressure.", 'Medicine used to treat high blood pressure does not itself reduce stress.']