问一道假定题

Although the ratio of physicians to total population is about the same in the United State 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 Candadians.

The explanation given above rests on an assumption that

A. patients in the United State do not have a greater need for surgery than do patients in Canada.

B. the population of 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 rule examine a patient who is a candidate fore surgery before sending the patient to a surgeon.

E.there are no unnercessary surgical operations performed in Canada

答案是A。

我不明白，题目中说，美国多33%的外科医生，还多了40%的手术，怎么还假定美国人做手术的需要反而少？不是需要应该多吗

原文的原因是：美国比加拿大人均多出33%的外科医生比例

结论是：上述原因是导致美国比加拿大人均经历更多40%手术的原因；

前提就是不是别的原因导致美国人比加拿大人人均动的手术多；而a很好的解释了导致上述结果的原因不是因为美国人比加拿大人需求多

能不能用数学模型推理？？