We can see that the author concludes "a good Filet-O-Fish is hard to find” by: C. Identifying the cause for the sandwich's rise in cost.
Conclusion actually refers to the final decision or judgement that one reaches after arguments and evidences have been thoroughly presented and evaluated. Conclusion helps one to understand what the whole point made is all about.
We can see that as the author concludes with "a good Filet-O-Fish is hard to find”, he is pointing out and identifying the cause for the sandwich's rise in cost.
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