Because visual inspection cannot reliably distinguish certain skin discolorations from skin cancers, dermatologists at clinics have needed to perform tests of skin tissue taken from patients.  At Westville Hospital, dermatological diagnostic costs were reduced by the purchase of a new imaging machine that diagnoses skin cancer in such cases as reliably as the tissue tests do.  Consequently, even though the machine is expensive, a dermatological clinic in Westville is considering buying one to reduce diagnostic costs.

Which of the following would it be most useful for the clinic to establish in order to make its decision?

A Whether the visits of patients who require diagnosis of skin discolorations tend to be shorter in duration at the clinic than at the hospital
B Whether the principles on which the machine operates have been known to science for a long time
C Whether the machine at the clinic would get significantly less heavy use than the machine at the hospital does
D Whether in certain cases of skin discoloration, visual inspection is sufficient to make a diagnosis of skin cancer
E Whether hospitals in other parts of the country have purchased such imaging machines

答案是D，是不是违反前提啊，前提说分不出来，请各位高手看看

Yeah…the ans is C..

谢谢大家，那我再问一下，D Whether in certain cases of skin discoloration, visual inspection is sufficient to make a diagnosis of skin cancer

是不是违反Because visual inspection cannot reliably distinguish certain skin discolorations from skin cancers这个前提

我说说我的想法

若只单看这句: visual inspection is sufficient to make a diagnosis of skin cancer与文章开头第一句visual inspection cannot reliably distinguish certain skin discolorations from skin cancers确实相左

但若加上in certain cases of skin discoloration, 则不能说它与文章前提相左

请指教