Which excerpt from "Mending Wall" best conveys the poem's theme?

A."We keep the wall between us as we go."

B."Good fences make good neighbors."

C."Before I built a wall I'd ask to know / What I was walling in or walling out . . ."

D."We have to use a spell to make [the stones] balance: / 'Stay where you are until our backs are turned!'"