Well for this ques. you need to find something that is different one some areas than other because even if there is something wrong going on and that thing is going on everywhere than there is NO reasoning that VARIATION will be there.

B clearly says that the variation is NOT due to factors that could affect surgery so it strengthen the conclusion that variation is due to unrelated factors.

C is wrong because, as OG mentioned, we are concerned with only the mentioned procedure BUT even if you do not see that point C can be eliminated because if unnecessary surgeries are stated in general and are performed everywhere so WHY there is VARIATION(this is point i stated in 1st paragraph above).