OG CR172

OG CR172よろしくお願いします。

There is a great deal of geographical variation in the frequency of many surgical procedures -- up to tenforld variation per hundred thousand between different areas in the number 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?

回答は(B)です。
(B)The variation is unrelated to factors (other than the surgical procedures themselves) that influence the incidence of deseases for which surgery might be considered.

argumentの内容から(B)の、「外科手術の変動は、手術を考慮すべき病気の発生に影響する要因と関係しない」というところにたどり着くロジックが理解できません。

わかりやすく説明していただけるでしょうか？
よろしくお願いします。