By default, word has a short horizontal line before the first reference. If you refer to diffusion, there is a long horizontal line in the diffusion. You cannot select or delete these rows. This is the sign of endnotes, but it is impossible to have such a line in the format of general scientific papers.

Switch to normal view and select View-Footnote from the menu. At this point, the edit field of the endnote appears at the bottom. Delete it, switch back to page view, and the horizontal line disappears.