xdf网络课件上的一道题，怎么也想不明白为什么是a

讲义 Page 11
例6． 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 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 thanthat 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.
正确答案是A，但是按照not+weaken的做法，a读上去是加强作用啊！

费允杰课件里说了一下，说什么是因果结果，我还是搞不懂为什么选a。

not+weaken法不错。