Respuesta :
A. Frost is explaining that one farm has apple trees which cannot move on their own, so they will never move forward to eat the pine cones under the neighbor's pine trees. If they had grazing animals it would be necessary for the fence to be there because it would stop the animals from crossing properties and eating off of each other's land, but they don't.
(APEX) A. The metaphor comparing apple trees to grazing animals emphasizes that the wall is unnecessary.