In Tim O'Brien's "Ambush," which element directly enhances the theme of the past's imprint on the present?

Bcold,less-dense air falls

The circumcenter is  equidistant  from each vertex of the triangle. the circumcenter is at the  intersection  of the  perpendicular bisectors  of the triangle's sides. the circumcenter of a right triangle falls on the side opposite the right  angle. the incenter  of a triangle is always inside it.