Sentence Correction (SC) | Re: Researchers have found that, on average, one American should be struck

I think D.
‘should be’ makes it seem like it is important, which is incorrect. So A,B,E are eliminated
In C and D, C looks correct however D has the term ‘every 13 days’ in the beginning which is the noun being modified by ‘on average’ which is non-underlined. C has the noun farther away than D. Therefore, D is correct.

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