There is a great deal of geographical variation in the frequency of many surgical procedures—up to tenfold variation per hundred thousand between different areas in the numbers of hysterectomies, prostatectomies, and tonsillectomies.
To support a conclusion that much of the variation is due to unnecessary surgical procedures, it would be most important to establish which of the following?
(A) A local board of review at each hospital examines the records of every operation to determine whether the surgical procedure was necessary.
(B) The variation is unrelated to factors (other than the surgical procedures themselves) that influence the incidence of diseases for which surgery might be considered.
(C) There are several categories of surgical procedure (other than hysterectomies, prostatectomies, and tonsillectomies) that are often performed unnecessarily.
(D) For certain surgical procedures, it is difficult to determine after the operation whether the procedures were necessary or whether alternative treatment would have succeeded.
(E) With respect to how often they are performed unnecessarily, hysterectomies, prostatectomies, and tonsillectomies are representative of surgical procedures in general.

Highlight below
To establish that much of the variation is due to unnecessary surgical procedures, it is necessary to eliminate the possibility that the geographical variation reflects variation in the incidence of disease treated with these procedures. Choice B, if established, would eliminate this possibility and is thus the best answer. Review boards (choice A) would provide some control against unnecessary procedures, so choice A would, if anything, tell against the suggested conclusion. Neither choice C nor choice E bears on the conclusion, since neither the conclusion nor the cited geographical variation involves procedures are of the kind choice D
describes, the difficulty of determining an individual operation’s necessity would merely increase the difficulty of verifying the suggested conclusion.

I don't understand this problem: what is the main point here? And what we try to break?

Sorry, but could you bring me some example so I can easily understand? What would be the definition of "variation" and "unnecessary" in this context?

Let me take a chance at this. I did this question a while back and got it wrong at first without an explanation

My choice this time B

Elimination
A - basically weaken the suggested conclusion. If a board review, then it should be necessary
C - out of scope
D - if the opeation is diffuclt to assess and determine, then we can never be sure. If we cannot be sure, it won't strengthen the argument
E - out of scope and somewhat a pure statement rather than argument

By the way, is this a LSAT question. What is the source. This type are very tricky. Perhaps we can put together a list for the Club.