牛人们再问一道！

sunnysunny11

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 hava a greater need for surgery than do patients in the 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 no 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.
为什么选A啊？

sunnysunny11

谢谢大家了！

holacraig

我的理解是，美国人均外科医生的比率比加拿大高，这个原因得出美国人人均接受外科手术的比率也高。而前提是人均接受外科手术比率高不是由于美国人对手术的需求高而引起的。非N人，希望大家补充。

sunnysunny11

有道理，谢谢啊。

gracezz

题干里把高出来的40%的外科手术率归因于USA外科医生比加拿大外科医生的数量多30%。

assumption需要排除其他因素使结论成立，要“搭桥”（牛牛们都用这个述语，学来滴）

其中一个他因就是外科手术率高是因为实际有手术需要，而非医生多了手术就多了，取非结论不成立。