The MLA style specifies that a superscript be used for a note reference mark to signal an explanatory note exists either at the bottom of the page as a(n) __________.

A. footnote or at the end of the document as a startnote
B. headnote or at the end of the document as an endnote
C. footnote or at the end of the document as an endnote
D. headnote or at the end of the document as a startnote