In many questions asking you to strengthen an argument, weaken an argument, or identify an assumption, there will be a shift in scope from the evidence (in this case, the first two sentences) to the conclusion (the third).
When there is such a scope shift, the answer choice will rest right in the space between them, linking the evidence and conclusion. More precisely, it will link the scope of the evidence to the scope of the conclusion.
Often, when you can’t identify a single scope for a CR passage, it isn’t because you don’t know how, it’s because there isn’t a single scope.
Scope, then, is limited to the conclusion, unless there is a shift in scope from the evidence to the conclusion. Err on the side of specificity: if an answer choice seems to be focusing on the wrong part of the question, even if you can make sense out it, it’s probably wrong.