CXD 逻辑第2篇30题 请牛人帮忙

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 patient who is a candidate for surgery before sending the patient to a surgeon.
E) there are no unnecessary surgical operations performed in Canada.

答案是A    不是很清楚为什么选A  如果A可以的话  那么B为什么不可以呢
谢谢各位大侠