He only says, "Good fences make good neighbors."

Spring is the mischief in me, and I wonder:
If I could put a notion in his head:
"Why do they make good neighbors? Isn't it
Where there are cows? But here there are no cows."
Before I built a wall I'd ask to know
What I was wailing in or walling out ,
And to whom I was like to give offense.
Which theme does this excerpt from the poem "Mending Wall" express?
a. walls can serve to keep things out as well as keep things in.
b. walls are only used to keep animals in a confined area.
c. everyone agrees that walls are useful.
d. all good neighbors agree with one another.

The correct answer is a. walls can serve to keep things out as well as keep things in.