19. Although the ratio of physicians to total population is about the same in the United States and Canada， the United States has 33 percent more surgeons per capita. Clearly， this is the reason people in the United States undergo 40 percent more operations per capita than do Canadians.

　　The explanation given above rests on an assumption that

　　（A） patients in the United States do not have a greater need for surgery than do patients in Canada

　　（B） the population of the United States is not larger than that of Canada

　　（C） United States patients sometimes travel to Canada for certain kinds of surgery

　　（D） General practitioners in the United States do not as a rule examine a who is a candidate for surgery before sending the patient to a surgeon.

　　（E） There are no unnecessary surgical operations performed in Canada.

　　Keys： AAECB BBDCA DBCDE AADA