One CR from OG11, don't understand OG's explanation, please give any comments on this CR, many thanks. 53．There is a great deal of geographical variation in the frequency of many surgical procedures-up to tenfold variation per hundred thousand people among 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． Argument Construction Situation The frequency of certain surgical procedures, e. g., hysteredctomies, prostatedctomies, and tonsillectomies, varies dramatically by geographical region. It may be possible to conclude that the disparity is to a large extent the result of the performance of unnecessary surgeries. Reasoning What additional information must be true for this conclusion to hold? Is it possible that different factors in different regions might reasonably account for the variation? Diseases or medical conditions for which these surgical procedures are appropriate might be more common in one geographical area than another．If the possibility of such geographical variations in the incidence of pertinent me(1ical conditions is ruled out，it is fair to conclude that the variation Can be attributed to unnecessary surgical procedures． The answer is B. | 
